Ubuntu. (blockchain of version 0.9.3 compatible with 0.10.1?)
In the official release announcement here:
https://bitcointalk.org/index.php?topic=1039713.msg11211433#msg11211433



Downgrade warning

Because release 0.10.0 and later makes use of headers-first synchronization and
parallel block download (see further), the block files and databases are not
backwards-compatible with pre-0.10 versions of Bitcoin Core or other software*



So upgrading works fine, but if you later decide to downgrade the block and database files won't be compatible.