I had the same issue, but I was able to still retrieve the messages via another apple device that was still connected to the iMessage service. I was then able t

I had the same issue, but I was able to still retrieve the messages via another apple device that was still connected to the iMessage service. I was then able to disassociate my number with the service. When I had my iPhone, I had originally linked both my email and my phone number to the same iMessage account, so fortunately I never lost messages. If it was tied to your email as well, you might be able to disable the service via another apple device.