User_ids were all changed

I used the imported and it successfully brought in all my users into my new site from a different site, but the users are now listed in alphabetical order and all have new IDs. This is not ok since I have many custom tables with references to those original user_ids. Is there a way to redo the import and keep the ids the way they were? Why on Earth would you do this unless there were conflicts between an imported user and an existing user?