I have had Paypal accounts since before it was even called Paypal and I have never been asked to change my password. I do so every now and then because it is a sensible precaution to take but only ebay have ever asked that I change all my passwords.

The only reason I know of for Paypal to ask you to change would be if something had happened that might make your account unsafe for instance you having fallen for a phishing email and that would always be onm an individual basis..