I have version 0.9.3 of bitcoind on my Ubuntu server and I want to upgrade to 0.10.1, I have nothing in my wallet. Does the blockchain 0.9.3 compatible with 0.10.1? Do I have anything to keep from version 0.9.3 ?
In the official release announcement here: https://bitcointalk.org/index.php?topic=1039713.msg11211433#msg11211433
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.