I would open a ticket under Account>Not listed here, and just write out an explanation of what happened, that or 'remove unwanted starter edition' and they'll remove the starter account (which is what they're going to end up doing anyway) and you'll simply want them to also transfer the character. Either of those options should work if you just write it out clearly.
Something like "Hello! a friend wanted me to play wow so I bought the game and created a character and we've been levelling together and having fun, but I now realize that I'm playing on a starter account I created years ago instead of my new paid for account. I'm very attached to the progress we've made together and I'm hoping you can help me with this situation
I really don't want to lose the character and I have a paid account I haven't really been using as you can see. Any help would be hugely appreciated."
Yes, I use emojis in my tickets, screw the haters. I mean, not saying to just copy paste that but that's close to what I would write, maybe a bit more detailed. I'd be SHOCKED if they didn't help you with something like that.