In any kind of distributed system there are failure modes you can't protect against, the best you can do is create a mechanism that will work in most cases and go with that.

When you leave you can export the people you follow, and it's likely that without too much trouble you can get the list of people who follow you. I've just done so, and it didn't seem to hard. There isn't really a simple interface for it - I just listed them and saved the HTML, then extracted them from that.

Having got your list of followers you can then ping them to let them know you've moved, and I suspect someone somewhere is working on a way of doing that semi-automatically.