Existing subscribers get six months of the PDF, and for the time being will be able to download old articles from its archives, but that isn't going to continue indefinitely.
SJG say that the new magazine will contain at least as much content as four weeks of the old webzine, but I doubt that the mix will be the same. princeofcairo has already said that he will not be carrying his Suppressed Transmission column over to the new magazine - for me that was one of the main reasons to keep subscribing to the web zine. I think it's possible that the non-GURPS content of the magazine, already low, will disappear completely.
Hopefully I'm worrying unnecessarily, and the new version will still be worth the subscription. But with the dollar on the rise compared to sterling and the Euro they may find that they're losing a lot of their European audience six months down the line. Anyway, I wish them all possible success; I'm just not convinced it's a wise move.
As part of this restructuring the SJG newsgroups (currently only accessible if you were a Pyramid subscriber) will also close, replaced by bulletin boards. As far as I can tell these will continue to be free. Not quite as convenient for me as newsgroups, but on the whole not a bad thing I suppose.