> need to coordinate a much more cohesive campaign that reaches customers directly

I think this is the crux. You should definitely mail everyone affected.

Send a letter 2 years before. 1 year before. 1 month before. Do a scream test at termination date (switch off for a day and see who pipes up), keep operating for two weeks and that's it.

Some people will never switch as long as "it works", but that's not your problem. Some letters, a website that the young generation can easily share (a daughter visits and a son takes care of IT, not uncommon.)