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 one midrange needs a new foam surround, should
they all be done, on the theory that if the foam of one has deteriorated, the are all probably in similar shape? there are 4 (d'appolito array) in my 14-year-old speakers.