A succinct question: does Sonar still support Garritan as a plugin? (as an "instrument" or a "synth" or whatever?)

"The Garritan [Personal Orchestra 5] sound library is AAX, VST, and AU compatible", so the answer's yes, I think.

Yes it does. I have GPO4, GPO5 and GIO with Aria player. All is well.

Thanks for the quick replies.
I have an older version of GPO (which I haven't installed on my new[er] computer), and the upgrade price is very attractive...

It's definitely worth the price. The sounds I've heard in GPO5 are good and with some tweaking some of them are really good but nothing mind-blowing. There are much better libraries out there but they also cost a lot more money. For $150 bux you'll get some nice samples. Definitely better than anything in the Garritan Pocket Orchestra.

Garritan told me via support email that the upgrade price on GPO5 was only available to GPO4 users and that my only option to upgrade my very old GPO1 was a new full purchase, has that changed?

I bought GPO4 2 days before GPO5 was released which automatically entitled me to a free upgrade to GPO5. They really should offer all registered GPO owners some sort of discount no matter how old the software is IMO. 

msorrels
Garritan told me via support email that the upgrade price on GPO5 was only available to GPO4 users and that my only option to upgrade my very old GPO1 was a new full purchase, has that changed?

I got the same response.  Can't do GPO3=>GPO5 at discount.  If that changes, would appreciate hearing.