I just called Apple Card and asked about a laptop purchase I just made a few days ago. The only option was to return it and buy it again to get the 6% instead of the normal 3%. Given that it would be $80, it kinda sucks but it would be a lot of time to sink.
I wish it applied to all Apple purchases, though. Most of my subscriptions run through Apple, so I initially got excited, then read the fine print. Oh, well.
So right now the Apple Card purchase options are 6% back on outright purchases, 3% back on the monthly payments in the iPhone Upgrade Program, or - rolling out today - 0% interest over two years on monthly payments for purchases.