I can't see a specific answer to your question in the CC FAQ, but it seems sensible that you could cancel your current plan, and take out a full subscription. If you are tied into a year's single app subscription, then my best guess is that you would need to make the full subscription a year's commitment as well.
But since they introduced the bargain priced Photography deal for Photoshop CC and Lightroom 5 of $10/month, $50/month for a full subscription feels just a tiny bit steep IMO. It would have been nice if Adobe extended the same good will to pro customers with say, a $40/month deal.
Yes, you can do that very easily. I first subscribed to the PS/Lr deal, while still using the CS6 master collection for the other apps. Then a few months later I upgraded to a full CC subscription.
I did it through chat, and the whole deal was up and running and functional inside ten minutes. All I had to do was sign out of the CC app and back in, and then it was all there. What they did was just cancel my existing subscription and sign me up for a new one. It didn't cost me anything extra.