Index: src/java/org/apache/lucene/index/CheckIndex.java
===================================================================
--- src/java/org/apache/lucene/index/CheckIndex.java	(revision 965112)
+++ src/java/org/apache/lucene/index/CheckIndex.java	(working copy)
@@ -347,8 +347,8 @@
       sFormat = "FORMAT_DIAGNOSTICS [Lucene 2.9]";
     else if (format == SegmentInfos.FORMAT_4_0)
       sFormat = "FORMAT_FLEX_POSTINGS [Lucene 4.0]";
-    else if (format < SegmentInfos.CURRENT_FORMAT) {
-      sFormat = "int=" + format + " [newer version of Lucene than this tool]";
+    else if (format < SegmentInfos.CURRENT_FORMAT || format > SegmentInfos.MINIMUM_FORMAT) {
+      sFormat = "int=" + format + " [newer version of Lucene than this tool, or pre-3.0 index]";
       skip = true;
     }
 
Index: src/java/org/apache/lucene/index/codecs/DefaultSegmentInfosReader.java
===================================================================
--- src/java/org/apache/lucene/index/codecs/DefaultSegmentInfosReader.java	(revision 965112)
+++ src/java/org/apache/lucene/index/codecs/DefaultSegmentInfosReader.java	(working copy)
@@ -41,7 +41,7 @@
       int format = input.readInt();
   
       // check that it is a format we can understand
-      if (format < SegmentInfos.CURRENT_FORMAT)
+      if (format < SegmentInfos.CURRENT_FORMAT || format > SegmentInfos.MINIMUM_FORMAT)
         throw new CorruptIndexException("Unknown (newer than us?) format version: " + format);
   
       infos.version = input.readLong(); // read version
Index: src/java/org/apache/lucene/index/codecs/preflex/SegmentTermEnum.java
===================================================================
--- src/java/org/apache/lucene/index/codecs/preflex/SegmentTermEnum.java	(revision 965112)
+++ src/java/org/apache/lucene/index/codecs/preflex/SegmentTermEnum.java	(working copy)
@@ -41,6 +41,9 @@
 
   // NOTE: always change this if you switch to a new format!
   public static final int FORMAT_CURRENT = FORMAT_VERSION_UTF8_LENGTH_IN_BYTES;
+  
+  // when removing support for old versions, levae the last supported version here
+  public static final int FORMAT_MINIMUM = FORMAT_VERSION_UTF8_LENGTH_IN_BYTES;
 
   private TermBuffer termBuffer = new TermBuffer();
   private TermBuffer prevBuffer = new TermBuffer();
@@ -78,8 +81,9 @@
       format = firstInt;
 
       // check that it is a format we can understand
-      if (format < FORMAT_CURRENT)
-        throw new CorruptIndexException("Unknown format version:" + format + " expected " + FORMAT_CURRENT + " or higher");
+    if (format > FORMAT_MINIMUM || format < FORMAT_CURRENT)
+      throw new CorruptIndexException("Incompatible format version: " + format + "; expected in range " +
+                                      FORMAT_MINIMUM + ".." + FORMAT_CURRENT);
 
       size = input.readLong();                    // read the size
       
Index: src/java/org/apache/lucene/index/FieldInfos.java
===================================================================
--- src/java/org/apache/lucene/index/FieldInfos.java	(revision 965112)
+++ src/java/org/apache/lucene/index/FieldInfos.java	(working copy)
@@ -40,6 +40,7 @@
   public static final int FORMAT_START = -2;
 
   static final int CURRENT_FORMAT = FORMAT_START;
+  static final int MINIMUM_FORMAT = FORMAT_START;
   
   static final byte IS_INDEXED = 0x1;
   static final byte STORE_TERMVECTOR = 0x2;
@@ -307,7 +308,7 @@
   private void read(IndexInput input, String fileName) throws IOException {
     format = input.readVInt();
 
-    if (format > FORMAT_START) {
+    if (format < CURRENT_FORMAT || format > MINIMUM_FORMAT) {
       throw new CorruptIndexException("unrecognized format " + format + " in file \"" + fileName + "\"");
     }
 
Index: src/java/org/apache/lucene/index/FieldsReader.java
===================================================================
--- src/java/org/apache/lucene/index/FieldsReader.java	(revision 965112)
+++ src/java/org/apache/lucene/index/FieldsReader.java	(working copy)
@@ -104,9 +104,9 @@
       
       format = cloneableIndexStream.readInt();
 
-      if (format > FieldsWriter.FORMAT_CURRENT)
-        throw new CorruptIndexException("Incompatible format version: " + format + " expected " 
-                                        + FieldsWriter.FORMAT_CURRENT + " or lower");
+      if (format < FieldsWriter.FORMAT_MINIMUM || format > FieldsWriter.FORMAT_CURRENT)
+        throw new CorruptIndexException("Incompatible format version: " + format + "; expected in range " +
+                                        FieldsWriter.FORMAT_MINIMUM + ".." + FieldsWriter.FORMAT_CURRENT);
 
       fieldsStream = (IndexInput) cloneableFieldsStream.clone();
 
@@ -185,11 +185,9 @@
   }
 
   boolean canReadRawDocs() {
-    // Disable reading raw docs in 2.x format, because of the removal of compressed
-    // fields in 3.0. We don't want rawDocs() to decode field bits to figure out
-    // if a field was compressed, hence we enforce ordinary (non-raw) stored field merges
-    // for <3.0 indexes.
-    return format >= FieldsWriter.FORMAT_LUCENE_3_0_NO_COMPRESSED_FIELDS;
+    // Since we currently only support >3.0 format anymore, always return true!
+    // I leave this method in because it may help for later format changes.
+    return true;
   }
 
   final Document doc(int n, FieldSelector fieldSelector) throws CorruptIndexException, IOException {
@@ -306,7 +304,6 @@
 
   private void addField(Document doc, FieldInfo fi, boolean binary, boolean tokenize) throws CorruptIndexException, IOException {
 
-    //we have a binary stored field, and it may be compressed
     if (binary) {
       int toRead = fieldsStream.readVInt();
       final byte[] b = new byte[toRead];
Index: src/java/org/apache/lucene/index/FieldsWriter.java
===================================================================
--- src/java/org/apache/lucene/index/FieldsWriter.java	(revision 965112)
+++ src/java/org/apache/lucene/index/FieldsWriter.java	(working copy)
@@ -39,6 +39,9 @@
   // switch to a new format!
   static final int FORMAT_CURRENT = FORMAT_LUCENE_3_0_NO_COMPRESSED_FIELDS;
   
+  // when removing support for old versions, levae the last supported version here
+  static final int FORMAT_MINIMUM = FORMAT_LUCENE_3_0_NO_COMPRESSED_FIELDS;
+  
     private FieldInfos fieldInfos;
 
     private IndexOutput fieldsStream;
Index: src/java/org/apache/lucene/index/SegmentInfos.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentInfos.java	(revision 965112)
+++ src/java/org/apache/lucene/index/SegmentInfos.java	(working copy)
@@ -67,6 +67,9 @@
   /* This must always point to the most recent file format. */
   public static final int CURRENT_FORMAT = FORMAT_4_0;
   
+  /* This must always point to the first supported file format. */
+  public static final int MINIMUM_FORMAT = FORMAT_DIAGNOSTICS;
+  
   public int counter = 0;    // used to name new segments
   
   /**
Index: src/java/org/apache/lucene/index/TermVectorsReader.java
===================================================================
--- src/java/org/apache/lucene/index/TermVectorsReader.java	(revision 965112)
+++ src/java/org/apache/lucene/index/TermVectorsReader.java	(working copy)
@@ -36,6 +36,9 @@
 
   // NOTE: always change this if you switch to a new format!
   static final int FORMAT_CURRENT = FORMAT_UTF8_LENGTH_IN_BYTES;
+  
+  // when removing support for old versions, levae the last supported version here
+  static final int FORMAT_MINIMUM = FORMAT_UTF8_LENGTH_IN_BYTES;
 
   //The size in bytes that the FORMAT_VERSION will take up at the beginning of each file 
   static final int FORMAT_SIZE = 4;
@@ -186,10 +189,9 @@
   private int checkValidFormat(IndexInput in) throws CorruptIndexException, IOException
   {
     int format = in.readInt();
-    if (format > FORMAT_CURRENT) {
-      throw new CorruptIndexException("Incompatible format version: " + format + " expected " 
-                                      + FORMAT_CURRENT + " or less");
-    }
+    if (format < FORMAT_MINIMUM || format > FORMAT_CURRENT)
+      throw new CorruptIndexException("Incompatible format version: " + format + "; expected in range " +
+                                      FORMAT_MINIMUM + ".." + FORMAT_CURRENT);
     return format;
   }