We are migrating web host and I wonder if it is possible in the migration phase to do a CNAME forward from
http://address2.com. Would the certificate still work even if the new domain doesn't have SSL yet?
A CNAME record does not "forward" anything, nor does it have anything to do with either HTTP or HTTPS protocols.
A CNAME record basically tells your nameserver that the record is an alias for another (A) record and it should lookup the value for that. For example:
Would make your nameserver query
If you want to redirect from one domain to another, with a different protocol, you will have to instruct either your webserver or the visitor's browser to redirect the request. There are some ways to do that.
Yes it's possible to do the forward, but it won't work, you'd need a new certificate as @paulmorriss points out in the comments.