What are the flaws (if any) in this approach?
If the current ones are designated as invalid a new design is required should they come back that is different from this to avoid impersonation. This would add extra cost and time should the project get resurrected; if they aren't invalid the roll-out can occur with greater speed using the old design...
It's very common for countries to have two concurrent designs of ID cards valid for international travel, so I don't think this is a problem. Look at Germany for example.