The blockchain for Bitcoin clients is going to split off from older clients on May 15th and the final warning has already been put into electronic ink by the maintainers (and the community in general.) To the uninitiated, this sounds a lot more ominous and terrifying than it really is, but really this sort of controlled for is more or less like an older version of the protocol being made obsolete so that a new version can take over. If your current Bitcoin client is up-to-date, you don’t have to do anything; if it’s not, upgrade it now.

Just two months ago we saw a much more technically problematic fork that was caused by a disagreement between v0.7 and v0.8 clients. The technical details of this problem rested primarily in that the earlier client was rejecting larger blocks being produced by a new client; to fix the problem the majority of clients were downgraded to the earlier v0.7 client (and the v0.8 clients changed to make blocks that the earlier version could understand.) This solution has only been temporary, however, and eventually Bitcoin clients are expected to upgrade.

That upgrade is coming on the 15th.

The Bitcoin Money blog describes the hard fork as thus, including information on upgrading and some technical details:

Released following the Bitcoin blockchain fork on March 11th 2013 was an updated v0.8 client (v0.8.1) that temporarily respects an undocumented limit found in all prior versions of the Bitcoin-Qt/bitcoind clients.

That temporary grace period ends and on May 15th 2013 (12:00 am UTC) any node that still hasn’t been fixed to remove this limitation will eventually find itself rejecting valid blocks from the Bitcoin blockchain.   It is possible that these unfixed nodes will see block confirmations but those blocks will not be part of the longest chain so it is critical that no commerce continue on or after May 15th using a node that hasn’t been upgraded.

Release v0.8.1 of Bitcoin-Qt/bitcoind has been available since March 18th, 2013 and accommodates the May 15th 2013 hard fork properly.   The fix has been backported to v0.7.3, v0.6.5, v0.5.8, and v0.4.9 clients as well.

For most Bitcoin users, this fork will have absolutely no effect on sending, receiving, or even holding onto BTC—but for those who haven’t upgraded their software in a while, it’s time to get that done. You have less than a day left to do so.

All software goes through upgrade periods and changes, and the Bitcoin infrastructure is a peer-to-peer network made up of everyone who uses a wallet, miners who authenticate trades, and the exchanges. For the most part a giant portion of the community has already upgraded (more than enough to make the fork happen without a hitch) and so little expected to happen on “Big Fork Day.”

[Image credit: REUTERS; A giant fork sculpture, created in 1995 by artists Jean-Pierre Zaugg and Georges Favre, is pictured in Lake Leman, Switzerland.]