So, I was thinking... Since a lot of people tend to associate users with their av/sig sets and a total change can throw a lot of people off and they don't even know who someone is that they 'knew' before because they no longer visually recognize them, I think an av/sig transitionary period is wise.
That is, change avatar and leave it for a few months so people still recognize your signature and get used to it, and then when you change your signature they will have already made the connection to your avatar so you'll never be the recipient of being forgotten based on visual recognition.
*nods to self*