You'd have to check with the baker, but from what I understand X3 (basic) is the same engine as X3 Pro except for the Prochannel and "extras." I've never had a problem laying over one version of SONAR over another, but I've never stepped up from basic to Pro. I dont' know how much overwriting goes on. If you save a restore point now, you can try to lay it over. If that doesn't work, restore and try after removing basic.
But I would try to talk to someone at cake. eVen the sales force could put you in touch w/ a baker answer.
@