So I assume us "lifetimers" that bought Platinum will be able to sue SOME of the goodies that came with Sonar that we ended up expanding or paying extra for.

I assume AD2 and any adpacks we bought will transfer.

What about the softsynths like Z3ta+, Rapture etc?

Melodyne?

Any others folks think of?

If they're integrated within Sonar they won't work elsewhere.  If they're VST there should be no issue....

All the standard issue Cakewalk plugs will work. Only those under special license, like Melodyne, BREVERB, TH2 & 3, etc. won't.
 
In fact, they only work in the Sonar version (and beyond) they were offered in. For example, BREVERB won't work in Sonar X1 or prior.

If you updated 3rd party vsts they should work - the Overloud stuff and AD2.