Nice to hear you solved the issue.
If anyone stumbles upon this in the future, I'd say there shouldn't be any issue if you modify your database offline, for example after importing data but before launching Flarum. IDs can be changed and AUTO_INCREMENT can be set to whatever value.
Once the forum is live, changing user IDs in production is risky. Not only the app could break, some relationships might not cascade automatically, but most importantly user IDs are used in sessions. So if you do ever change IDs on a production server, you should clear all PHP session from the server memory. otherwise if an ID matches another user, people will end up connected to someone else's account.