X3 is the previous release of Sonar.
The current release is simply called Sonar in 3 flavours, Artist, Professional, Platinum
Your projects should open but plugins may present problems as you will need the same ones in your new Sonar product
Seeing as this version 7 of Sonar is not registered to you you may have a problem getting the upgrade discount as technically Sonar is not permitted to be resold by Cakewalk, nor do Cakewalk do licence transfers.
If you look n the website you'll find a comparison showing feature comparison between the current versions
Since Sonar 7 there's been 8, 8.5, X1, X2, X3 and now the new Sonar release this year. That's a total of 6 versions and the program has changed significantly, it has many more functions, extra content, a completely different workflow but is much faster, stable and better suited to fulfilling its job as a DAW.