if one midrange needs a new foam surround, should

Agree with Stanwal. Besides physical condition, it is best to re-foam both sides at the same time to maintain balanced sound quality.

I say change only one and leave the other alone. The difference in compliance between the woofers could create a phasiness that will sound like a vintage analog flanger effect. With a little substance abuse it will sound way cool.

If the foam has not deteriorated to the point of falling apart, meaning it feels week but no holes in them, you can revive them for a couple of years with clear latex. Take some normal latex paint that has been sitting around, Do Not Shake it, Open it carefully and you will find the latex has seperated from the pigment. Carefully paint on Latex without color smoothly and carefully with a small brush. Let the first coat dry before applying any more. But you will find the foam soaks up the latex and does quite a good job reviving it. I came from the old SpeakerCraft/Marcof Electronics out of Webster Groves Missouri. I have done this 100 plus times, it works well.
Good Luck, Tim

I agree with having drivers redone to maintain balance left to right. Even though the foam isn't deteriorated it is not as firm as it once was dramatically affecting the ease of excursion and overall sound character.