Change database value for a thread's "forumid" at threadfpdata_presave hook location?

[tweakmonkey]

What I'd like to do is change what forum a thread saves to after the user has already submitted a message (but before it's saved to the database I guess) based on certain criteria in the threadfpdata_presave hook location.

The part I'm stuck at is how could I modify this value?

For example, a user posts a thread in forumid=4, but because of some criteria (postcount, usergroup, etc.) it posts the thread in forumid=6 instead.

Would this need to take place in another hook location, like threadfpdata_start / doset?

What command would change the forumid value on the fly?

I'm new to this stuff and any links would be appreciated or explanations of what these hook locations are, mean, are listed, and so-on... Thanks!