If the twenty year old balls have been in a pool hall it is a nonissue, swap for new ones. The reason is simple. A big part of the price you pay for pool balls comes because they have been matched by size and weight, maybe even roundness. Life in a pool hall, the ball sets get mixed. There goes all of those tight tolerances, same thing if a ball or two comes up missing in a set and is replaced.
I bought a thirty dollar set of pool balls planning to cut them up for other projects. Lost interest in the other plan and eventually took the balls to a quality pool hall. Much to my surprise those balls played a lot better than the house balls that did get cleaned and polished regularly, two sets at a time! Looking at the "set" from the counter I could see three different designs so it was balls mixed from at least three sets, maybe more. Made me remember an old road player that always tried to sneak his balls on the table, failing that, at least his cue ball. If he couldn't get at least his cue ball in play he often didn't play.
A new set of balls won't have seen trips to the floor that can add deep scuffs or possibly affect roundness. Many considerations all favoring new balls. Worth cleaning the old ones, you may be happy with them then. If you still want new balls the cleaner and polish won't be wasted.
Good luck whatever you do. Keep us in the loop and let us know results!
Thanks,
Hu