Complete Collector's Edition Upgrade with full Complete Collector's Edition possible?

[Yeondan]

Not if you apply the CE code to your already-existing SquareEnix account under the already-existing FFXIV service account!

I'm assuming from the use of € that you're looking at the EU SE store, so the game code will be a EU game code.
Is your SE account EU region? The SE account region and the game code region need to match.
Is your current license PC, PS4, PS5, or Steam? Steam licenses are incompatible with normal PC game codes.
Don't create a new SE account when you register the game code.
Don't create a new FFXIV service account when you register the game code.

So, I just bought the Collector's Edition and tried redeeming it on my already existing account but it says the code is invalid. I bought it on the EU store as you told me. Do I have to redeem it somewhere else? Or is an upgrade from Complete to Complete Collector's Edition simply not possible?