The fact is that unless you want to continue to provide DNS forever, you will have to give those, who are slow to move, a drop-dead date. Otherwise, some will never do it on their own. You will need give them plenty of time, and keep the old server up, otherwise, you will have many pissed off customers.
After moving the ones you control, if there are not too many, I would give the others a two-month warning (longer if you need more time to do what follows), and tell them that you will start calling them to ask them if they need help with the process.
You made a major change in that you can't just point your ns.nameserver.com to a new IP, so they deserve some extra help if they need it, in my opinion.
