Yoma 37 Changing the כיור

The vessels in the Mishkan had an exact measurement explicitly delineated in the Torah such as the Mizbeach, Shulchan and Menorah. These keilim could not be changed ever. What about the other ones such as the kiyor? Did they have to be made in the same way they were made in the times of Moshe? Rashi in Chumash says that they must and the Ramban there disagrees and holds that they could be made in any way they wanted and he prooves it from the Mizbeach Hanechoshes that was built to a much larger size in the times of Shlomo Hamelech. Look in the Mikdash Dovid (2) where this issue is discussed.

Our Mishna states that Ben Katin changed the amount of spouts that were on the כיור. Originally there were just two and he instituted that there should be twelve. How would Rashi explain the permissibility of this? Is that not considered a change?