how can I do this

[Xenon]

nope there isn't as it's something which is not really needed

if you want to do so you have to run a lot of queries:

[sql]UPDATE table SET forumid = 1 WHERE forumid = 48[/sql] for every table which has a forumid field.

not that you ahve to update all caches as well afterwards, so i'd advice you not to do so