No no no. At least up until recently, CA law is different from MA law in that regard. I believe that you have been allowed to modify post-ban magazines down to 10-rounds or less and that has been legal. There is no specific provision in MA law to allow post-ban magazines if they are modified to reduce capacity.

But again, logic tells you it shouldn't matter when the magazine in question was originally manufactured as a 10-round magazine. The problem is that badly written laws are often misinterpreted when it benefits the anti-2A agenda.