Are you should marketing would say no. I would think they could make a small fortune selling different type Pikachu plushies and other merchandise. There is no reason they still couldn't keep the original as the mascot.
Yeah, and they do already. Pikachu has seen numerious forms that involve costumes, but the core TM needs to stay consistent for the branding. They could do a special costume version, or change the size of pikachu, but a major core design change could be problematic and confusing to the brand recognition as a whole.