Sorry, but you can't use the CE version of the game if you already the base game on the same platform, it's for new service accounts only or for service accounts that don't yet have that platform (i.e. they have PC, but not PS4).

So if you have it for PS4, buying CE version also on PS4 will only work for creating a new service account (new characters, new subscription). The code can't be used to upgrade an existing account.

To upgrade you have either two options:

A) Do the digital upgrade via the Mogstation
B) Buy the collectors version on another platform (i.e. PC)

Buying on another platform will not only allow you to login on that (non HW areas unless you also buy that too), but will give you the CE items as well as 30 days free (because each new platform added adds 30 days).

Hope this helps to explain!