Changing the user
IDs could be problematic. Other tables reference those - including
wp_usermeta but if they are authors, customers, etc. then any tables that hold those items - like
wp_posts - reference the author by user
umeta_ids should be safe, because that value isn't referenced elsewhere that I'm aware of. It wouldn't hurt to check your particular plugins and other tables to see if they are indeed referenced somewhere.
It would be safest to use a plugin to migrate the users. It does appear that you'll have to have your old users reset their passwords, but that's a smaller price to pay than getting customers, authors, etc. all confused.
A couple of possible options: Importing Users into new WordPress site and importing users where password is provided as md5 + much metadata.