Back in the mid 90s the account number in DL's FFP was defined as char(10). Nothing to prevent the definition redefined as char(12) or even integer.

The larger issue is overlap but that is not a big challenge either. Keep the non overlapping numbers and assign a new number to one of the unfortunate individuals with duplicates.

Then someone has to decide which is more important; date of membership or acquiring company.

Or just let everyone use SSN. No overlap and I'm sure TSA would love having that available.