Redirect Options for retired hostname using SSL

Cool. Already you said 1st option to renew cert & redirect old url to new, So browser cache will store. Flash user to update bookmark.

2nd option is to Create alias of "hostname.olddomain.com" in New URL "hostname.newdomain.com" in DNS Level and assign SAN certificate name of both URL. Traffic impact will be less.

How To Generate SAN - https://support.f5.com/csp/article/K13471

In 2nd option your OLD URL(hostname.olddomain.com) IP will be free & all others resources can be utilized in other application.