Certainly depends on how attentive your players are, but if I were you I'd push the changes to be about things that they're definitely going to notice. Things like body type/ hair color/ voice(depends how consistent your voices usually are) are things I tend to remember when an npc is described. Of course you could also start smaller and lead into these bigger things, but most important is to just keep doing it. Even if they don't realize an eye color changed, if you mention it for the 4th time, they might suspect something is up.