I actually *just* did the exact same thing, I nuked my freemium town finally and left a big message for people to add my premium account instead. I figure that within a week the most helpful neighbors of mine will have already seen the message and sent a request. I don't plan on turning a single neighbor away who sends a request from my old town...I hope at least a good 30-40 old neighbors will add me back! I have noticed though that a few have decided to just delete me and never sent a request to my premium...so maybe they didn't read the message or simply didn't care to add me again :(
Anyway, I do agree that it's a bit rude, but there are plenty of other good neighbors out there...