I am looking into this since it's important to have it in place before the work on
LUCENE-5618 (and LUCENE-5636).
I am not sure about the rules here: is it ok to apply updates to e.g. a 3.x or 4.0 index?
No, updating those indexes is not supported (we even suppress those codecs in tests), since those codecs did not take the segmentSuffix into account. We've decided that in order to use updatable DocValues, you need to index with 4.6+, or re-create the index if it was created with earlier versions. More so, old formats' consumers aren't shipped w/ Lucene anyway.
I'll look into adding those indexes to TestBackCompat – recreate them w/ few numeric and binary doc-values and then try to update them with newer code.