How to change user / group numbers in passwd/group?

[Eric <spamsink@xxxxxxxxxxxxxxx>]

I am in the process of moving the content of an FC2 server to F7.  In the 
process, I have to establish some new users with user and group numbers 501 
and 502.

Unfortunately, F7 in its infinite wisdom decided to assign 501 to user 
"tclhttpd" and 502 to user "dbmail" (whatever they might be).  (vigorously 
begging the question of WHY!!! in the world it did that...)  :-(

If I can't assign 501 and 502 to the users and groups they were on the FC2 
machine, a LOT of permissions problems will be created in a fairly large 
and complex database and file structure.

What is the "right" way to assign different numbers to existing users like 
"tclhttpd" and "dbmail"?  I know I can just go in there and edit passwd and 
group but no doubt that will cause all sorts of unknown problems elsewhere, 
for resources and files that depend on 501 and 502 being assigned to 
"tclhttpd" and "dbmail".  So, is there a "right" way that updates 
everything else to whatever new user numbers I assign?