Let's say I'm now upgrading from X2 Pro to X3 (basic), due to budgeting issue, but I would like to use some of the improvements immediately, for the comp recording, etc.. (I can still use X2 Pro for mixing if needed).
 
Later when I'm ready, can I still upgrade to X3 Pro ?
 
Thanks!

I've just written to Sweetwater, they are very helpful and provided prompt replies.
 
It seems one cannot upgrade more than once... and the advise is it's better for me to save up and reserve my X2 Pro to be upgraded to X3 Pro.
 
However, I've noticed I've got a few Sonar LE license lying around unused (from Quad Capture and Juno-G). Further query confirms that I can use the LE license to qualify for the X3 (basic) :
 
http://www.sweetwater.com/store/detail/SonarX3upAC/
 
Cheers.