Update. I sent them a letter, notifying them of my change of address. I gave them my old address, old (MA) driver's license number, old ltc A number, and that was it.
Got a call today from the Dept. Of firearms records, which I didn't pick up, anticipating that they'd leave a message. They left no message. So, I called back, interested to see what they wanted, and was informed that it was the law to fill out and send the form with my new address. I informed the gentleman that he was wrong, that I'd read the law, and that the only requirement is a certified letter informing them of any change of address, with no requirement mentioned of a new address having be provided, which is especially true now that I'm no longer a MA resident. His authority as a very powerful government ruler must not be challenged often, because he became upset, and said he wasn't going to argue with me.
I asked how one could simply cancel their license, as if I was still in state, had sold all my guns, and didn't want to keep the license active any longer. He said to contact my local PD. So, I suppose I'll give that a shot tomorrow.
Edit: for everybody making the point that they could find me if they wanted, I have no doubt that's true. However, this is a matter of principle. MA can f*** right off with their bullshit, and if my noncompliance with their unlawful "laws", per the constitution, wastes enough of their time that even one of these people thinks, "maybe we've gone too far", I guess that'll be some kind of win.