I am considering moving to a new domain. In doing so would ALL users, even users that have stayed logged in on the previous domain, have to log back in to the site on the new domain? If so is there any way around it as I am sure I would lose a lot of users...thanks for your help

No, there is no way to avoid logging in again. Since the domain name changes, the old cookies won't work. There is no way around it.

The only way they may still work is if you move to a subdomain rather than a completely new domain.

Thanks Mate!