Well I'm still going to reply because I'm intrigued and confused after reading both this and the original post.
How can an entire production have a "SAG/AFTRA rate" anyway? That union covers on-camera and voice actors only, has no jurisdiction over any other aspects of the production.
But the bottom line, for any existing completed production that you own outright, when you sell it you can charge anything you want for it, obviously. I'm assuming you'd pay your talent additionally for the extra usage, but how much you do so is dependent on whatever original agreements were in place with them. If the buyer is using that production to craft a new production and using a SAG actor, only the SAG actor is under the jurisdiction of the union.
And by the way, you can't really depend on what SAG/AFTRA says anyway... as a producer they have absolutely no teeth to get you to do as they wish, no matter what they say... and they'll say a lot. They are also one of the most difficult and ornery organizations you'll ever want to deal with... and I say that as a card-carrying SAG/AFTRA member, myself.
Fantastic Plastic Entertainment, Inc. fantasticplastic.com
It's a very odd, unusual situation. SAG is required (Taft-Hartley) for the actors because of the new production company's involvement, as they are taking the old TVC and making a new one by using it and they are SAG signatory due to the new talent's involvement.
What I've been told by a couple of pros who work in the bigger agency world is, how they tend to deal with this is a new agreement with the old prodco, which involves them getting specific rights and paying a bit of money for said rights. The pitfalls to watch out for are plentiful, of course.