@polotek It seems like a poor architectural decision that the CDN URLs are embedded into posts in the first place. If the URLs in posts were `social.polotek.net/media/…` that should be very easy to redirect to a CDN (just redirect that whole media namespace) and then it could be changed out as necessary.
That would avoid the masto.host issue of complicating their initial setup with a subdomain, but make it easier to migrate later.