Deleting and re-adding email account is a pain, but manageable. Are you able to retain all the emails stored locally? This is my biggest fear when I upgrade. Losing years of email history would be a major issue for me.
Recently, under Lion, I had a Mail issue where all of my email accounts disappeared. The messages were still in the Library and re-adding the accounts using the same names "linked" the emails. No problems after a rebuild of the mail Spotlight index. Hopefully, this works for ML as well.
Rob