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?
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.
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.