Either your understanding of the law is incomplete, or mine is wrong. My understanding is that magazines holding more than 7 rounds are illegal, but there is a one-year "grace" period during which owners must either dispose of them or sell them out of state. During that one-year grace period they can be used if not loaded with more than 7 rounds.
So they're not really "grandfathered" at all.
I'm reading through the bill right now and the way it seems to me is that;
1) All "pre-ban" magazines capable of holding more than 10 rounds that were legally owned are now banned and the owners have one year to sell or dispose of them.
2) All magazines holding up to 10 rounds (that are not readily able to be modified to hold more rounds) are "grandfathered" in, however must not be loaded with more than 7 rounds at any time.
3) All magazines "obtained after the effective date of the chapter of the laws of two thousand thirteen" may not be able to hold more than 7 rounds and are not readily able to be modified to hold more rounds.
4) Exceptions to the aforementioned are tubular .22 rimfire magazines and;
a)curio and relic feeding devices manufactured no less than 50 years before the effective date that can only be used in firearms that meet the same age requirement.
All of the information is on page 32 of 67.