You should switch to a new pad if the pad you are using has been used with another product and has not been properly cleaned (with pad cleaner!) before using it with another product.
So say you just compounded your car with a white pad, you would not simply clean on the fly, and then continue to use that pad with a finishing polish. You would switch to a fresh pad that had been properly cleaned or is totally brand new.
Some people will say it is best to only use a particular pad for a certain product, even if it has been cleaned, however I think that is overkill... I would need a HUGE collection of pads. My feeling is that if you can properly clean the pads after each use so that they look and feel like new again, you can use any product with that pad for your next detail.