user27120: 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 ?

user208: It should be forward compatible, but making backups of your wallet is always recommended.
Once you upgrade to 0.10.x, downgrading to 0.9.x afterwards may not be possible anymore.

user27203: 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.

27120: <p>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 ?</p>
 208: <p>It should be forward compatible, but making backups of your wallet is always recommended.</p>

<p>Once you upgrade to 0.10.x, downgrading to 0.9.x afterwards may not be possible anymore.</p>
 27203: <p>In the official release announcement here:
<a href="https://bitcointalk.org/index.php?topic=1039713.msg11211433#msg11211433" rel="nofollow">https://bitcointalk.org/index.php?topic=1039713.msg11211433#msg11211433</a></p>

<hr>

<h2>Downgrade warning</h2>

<p>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*</p>

<hr>

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