Incremental User IDs?

I am in the process of upgrading my rt2 system to rt3. I noticed that when
I imported by rt2 database, the user ID’s changed. They are no longer in
the same order as they used to be. They have lost their incremental order
as well for new users. This is very bad for my system, as I have added some
custom code that relies on the user ID. Is their any reason that the user
Id have been changed? Or has there been an error in my installation, and
this is an intended change in the code.

Forrest Stanley