part of updating a DC from 4.2 to 4.3 consists in updating format of
storage domains from V4 to V5.
- where can I find list of differences between V4 and V5? And in general
between previous ones?
I'm afraid there's no design doc for it. Here's list of changes:
* domain metadata
* added: alignment, block_size
* removed: logblksize, phyblksize
* volume metadata
* added: cap (replacement for size, capacity is size in bytes, while size
was in mutiples of block size (512B))
* removed mtime, pool_uuid, size
If you want more details about the changes, you can check slides 
- can I force only a particular storage domain to remain in V4 while
the other ones upgraded to V5, for any reason? Or the reverse: update only
one of the defined domains?
I don't think this is possible.