That shouldn't be a bottleneck for Jagex, simply because every account has it's own ID in the database. The username will only refer to an ID, and doesn't have the role of the ID itself. In fact, your friend list and ignore list do only contain Account IDs in the database, once you login and view one of the lists, the usernames will be fetched. So you will still keep all your friends and foes in both lists, only the name might be not familiar with you. I hope that further debaters will keep that in mind.