I have X2 Producer and am considering upgrading to X3, but probably only Studio since I'm not that interested in AD and the AAS plugs.  Can anyone confirm if a newer Studio version will allow you to use Producer plug-ins (e.g. Pro-Channel, Breverb) from a prior version?

VST Plugins are generally compatible with all versions of Sonar.
 
By the way, up until yesterday you could have upgraded to Producer edition for the same price as Studio.
EDIT:
Maybe it's not too late after all. Right now it's still 31st March in Boston, USA. Not much time left though.

You can upgrade from X2 Producer to X3 Producer right now for about $84 from JRRShop.  Here's the thread with instructions, posted by cclarry:
 
http://forum.cakewalk.com/Upgrade-to-X3-Producer-from-X2-Producer-m3016564.aspx
 
Don't know how much longer this will be available though.

Yeah, saw the sale.  You could also purchase Studio for $49 which is the deal I was thinking about, but ended up doing producer after all.  Too bad I didn't see the JRR deal.
 
My concern though was specifically around producer exclusive plug-ins.  For instance, you can't run Breverb in 8.5 Studio because it tells you it's only licensed for X2 Producer.  So I did the producer upgrade just to be safe.