Hello guys. I own a copy of the original Vanilla CE and i would like to upgrade my account to have the companions. How can I do that?
Hey Gànker, this is the 
WoW italian’s forum and the topic will probably be closed.
Here for english: https://eu.forums.blizzard.com/en/wow/ 
I think you can’t redeem Vanilla Collector’s Edition code for get upgrade at your current account.
Q: Will my original WoW CE pets be available?
A: Yes—as long you’re playing on the same account that you created when you purchased the original WoW Collector’s Edition code.
So If you play WoW Classic on your account created when you purchased the original vanilla CE, you’ll get the CE pets.
I actually never redeemed that. No chance to do it now?
Hello Gànker
You should be able to claim a Collector Key via the “Redeem a Code” option available in your account management page.
If not please contact our GMs and they’ll be happy to check the situation for you