But would any of that functionality be lost if the subject was autofilled with "Old Subject" rather than "Re: Old Subject"?

Some email clients look for Re:.

(Personally I have never been a fan of comment subjects at all, because if someone fills it in, it overrides the default email notification subject that tells me whether it was a reply to one of my comments or just to A comment in a thread I am tracking. I'm probably the only one, heh.)