From what I understand, they're keeping the HD equipment turned on at the 99.5 TX site. Nassau might not be as much of an HD Radio supporter as Greater Media is, but they do have one other station running HD (WFKB 107.5 Reading PA) so I'd expect it to stay on.
Right now, 99.5's HD-1 channel is, of course, WCRB, with the PAD data reading just the words "Boston's Classical Station". They don't have an HD-2 up right now (the old WCRB's HD2 was the direct feed of the WCN), but I would expect that one is in the works.