Using replacement variables to adapt old BBcode?

[thefnool]

On my forum, until recently I was using YouTube bbcode with the following formatting:

[youtube=XXXXXX]XXXXXX[/youtube]

Where XXXXXX is the variable at the end of a youtube URL.

My new formatting is:

[youtube]http://www.youtube.com/watch?v=XXXXXX[/youtube]

However, with the change I am left with lots of posts containing unparsed embedded videos using the old code. I'm trying to work out a way to use replacement variables to alter every instance of the old formatting to the new formatting.

The way I see it, I need to globally replace [youtube=XXXXXX] with

[youtube]http://www.youtube.com/watch?v=

As this would mean that the closing [/youtube] tag would be left intact, and all old videos would then parse correctly.

What would the formatting be for a replacement variable that would replace all instances of [youtube=Anything goes here] with [youtube]http://www.youtube.com/watch?v= ?

If this is impossible, are there any plugins/products that would facilitate such a change? Or, is it possible to do a similarly specifically-targeted find/replace in my database?