Is it possible to update SOHO 5.9 preview to the deluxe edition through updates?
Is SOHO the only release with KDE ?
No it's not possible to update the preview through updates. Just doesn't work that way.
Yes, SOHO is the only release WITH KDE,....
but it's not the only release that KDE can be installed into. By that I mean it comes with a Very Nice KDE installed, but say if you have Standard, you can install KDE into that, but it's not quite the same as the KDE in the SOHO.