In my experience, we need to keep the SSL on the domain, as people who have saved pages will get a warning from their browser if they try navigating to the https page and it is no longer secure.

sounds like it'll work. just host the old domain on a linux account and use a .htaccess file to set up all the redirects, or php mod rewrite if you just want to redirect all urls to the same URI on the other domain. You can point both subdomains to the same hosting for the redirects in the DNS.

Thanks!