I've been SLOWLY moving from my old host to my new one. It's taken about 4 months though because of the number of subdomains and databases I have and the lack of time I also seem to have

I've finally moved ALL of my subdomains over to the new server. In this moving process however, when people using the new server would try to send email to me, it'd get delivered to the new server since it thought it was my mail host. I didn't bother changing that at the time figuring it wouldn't amount to much... Well it has. I need to migrate and merge the old email accounts into the new ones. What is my best bet for doing this? Any ideas? I want to finally get down to one host!