As gardnr says, you need to solve this problem by involving the clients. Trying to fix this without some DNS fiddling is going to waste your time and you'll have nothing to show for it.

The world changes, and adaptation requires maintenance. 10 years ago is almost half the age of the web ago. This is more a business decision - do you let those accounts rot - than it is a technical decision. Because the business decision needs to include support time for user interaction.