I am aware of the history there, so Paycent took preorders for Mastercard before it was integrated. Mastercard for some reason delayed and they are in negotiations with Mastercard to be able to issue them, last I heard was an ETA of a few weeks from now. At the moment you can only order Union Pay International cards which work in most countries, you can also order Union Pay China cards.

I understand they will refund or replace your Mastercard with a Union Pay card if you do not wish to wait for the Mastercards to be able to be activated.