I think you can do manual changes in the admin CP (User Group Promotions -> Manage Promoted Users). I believe you would have to demote them and then manually repromote them though.
We're a very old forum, some of our members achieved our highest ranks when we were on vBulletin. These ranks required a high amount of reputation to achieve, something that wasn't imported due to Xenforo's lack of a reputation system.
I would like to set other promotion requirements to achieve our high level ranks, but it would be unfair to demote our current members by adding additional criteria. Is there any other way to do this?
I think you can do manual changes in the admin CP (User Group Promotions -> Manage Promoted Users). I believe you would have to demote them and then manually repromote them though.
How about having 2 usergroups with the same permissions?
Leave the old members in the current usergroup and create a new usergroup with the new promotions setup.