Oh gosh, mini. Vanilla will spread and 'ordinary' users will try to use it. Some sql coders will do things like these in a blink of a eye or somebody could build on-the-shelf tools. I would preffer the latter as it is hard to find sql\ php coders and it is hard to explain what you actually want to accomplish. It will be hand-made and thus takes more time.
I was worried about same usernames in different forums but it could be manually checked. Not a problem as forums are small.
Comments
1. Unique keys are not duplicated.
2. All data relationships are maintained.
If you don't have a decent grasp of SQL and relational databases, I wouldn't attempt it.