how would i try upgrade from J-F? Need to call or?
I have no experience on that particular route. But it's the same as for a Y to J upgrade ex UK (where you have to pay additional APD), you can do it by 'upgrade using miles' on the website and it will then ask you for payment (by card). It's just the 'auto upgrade' that does not work as they need to interact with you to get the payment.
As an aside: if you upgrade on board in the UK Y to J scenario, you don't pay the additional APD, I have no idea how that would work on MLE/DXB