Do you have proof of any IDs being changed? Comparing with an old backup for example? If so, then it is more than likely an add-on.
My member have decals placed on there cars and they are not corresponding now... http://www.wickedstangs.com/community/members/v-i-massive.1594/
If this is an import and you had gaps in the user IDs in your old installation due to deleted members, etc. then that is why. Unless you checked Retain imported content IDs.
