Epoxy the mag base and that complies. Makes it a mess to clean the mags later though. Your other option is to buy 10 rounders and sell the 15's to someone out of state prior to the law going into effect.
I was in the process of getting a 9mm. I'm holding off until I see which way Christie swings.
I have 4 new in the bag NJ currently legal 15 round M&P9 mags. If this go thru I either have to block them or sell them to someone in CO.