Is it possible to upgrade my Digital Deluxe Edition (or whatever its called) to a Colletor's Edition, while buying the real deal and using its code on my account? Or does it mean I need to make a new account for it?
Yes you can. I did this exact thing, I bought the Digital Deluxe at launch, then I later bought the physical Collector's Edition, and redeemed the code in my account settings.
I got all of the extras which you would normally get with the CE (1 month game time, the extra Cartel Coins CE accounts got, and so on).