User and group are vpopmail and vchkpw. UID and GID are specified in your operating system's configuration file.On a lot of systems these days you can run "vipw" to edit the password file and change the UID.
Usually they are not in /etc/passwd but in another file, readeable only by root (master.passwd, shadow, something_else).
You must read you man pages about passwd.
vipw will update the shadow password file too (if you use shadow passwords).
On my newer Linux boxes, Ive seen a similar tool for groups called "vigr" which does the same
thing for /etc/group.
The original poster doesn't say what OS he's using so I can't help much more than that.
-- Aj. Systems Administrator / Developer