acid 2.0 is introduced in
HIVE-14035. It replaces Update events with a combination of Delete + Insert events. This now makes U=D+I the default (and only) supported acid table type in Hive 3.0.
The expectation for upgrade is that Major compaction has to be run on all acid tables in the existing Hive cluster and that no new writes to these table take place since the start of compaction (Need to add a mechanism to put a table in read-only mode - this way it can still be read while it's being compacted). Then upgrade to Hive 3.0 can take place.