“Never”
[Q: Will PAID transfers also be a thing should a server drop heavily in population and a player wishes to move to find more activity or reconvene with friends on another server? ]
A: We’re considering integrating a character transfer service once players have reached higher levels. This was available in the original WoW. We’ll look at the rules we imposed at that time and also discuss the implications of players moving to a new realm. We believe realm identity is an important aspect of Classic so any guidelines around transfers should support this belief.
I wouldn’t be so quick to claim that it will “never happen” if I were you, to be honest.