I happened to buy ap when it was on sale some months back, had I known tesla was going to release eap
I would have reconsidered the purchase...
Is there any discounts for people that have basic ap and want to upgrade to eap?
You have to have, or purchase, basic AP before you can purchase EAP. So no, there is no discount.
So it is not like you could have bought EAP for $4k has you not already purchased basic AP.