I would like to know if it's correct to use AP licenses, we bought and added on a WLC, on a new controller that will replace the old one.
I think I can do it as it's declarative on new WLC but is it correct from a Cisco point of view ?
I don't believe you can transfer licences between generations like that.
2504 is the same era as 5508/8510, whereas 3504 is the same age as 5520/8540.
You'd have to talk to your account manager, but I suspect you'll have to buy new licences for the new controller.
From as technical point of view you can do it. From a legal point however it breaches Cisco licensing unless you purchased the old APs with Cisco 1 licensing in which case they are transferrable.