Hey Thomas, dunno if this will help you in your case, but here’s the link to get Cubase SX3 from Steinberg…
Until someone chimes in, you can just try it. I’m still not sure you can actually use it with your SX2 license, you’ll have to get the word from someone at Steinberg, or a user who’s done it. I just realized that when I had tried the SX3 from the above download a long time ago, I had installed it on another computer with ‘no’ other version of Cubase, and ‘no’ dongle connected even…they were on my actual DAW machine.
Maybe missed it, but the link above doesn’t actually say WHAT license(s) you need to make this SX3 work 
A mod, or anyone else have that answer? That sure would be really nice of the people at Steinberg to just ‘give’ away SX3 to any Tom, Dick or Harry with ‘any’ Cubase version license…but I’m assuming you probably really need a Cubase version 4 license?