Apple will not help pay for any upgrades, nor can you at any point upgrade a part that is being replaced. For example, if they are replacing a broken HDD, you can not pay to have them install a larger HDD while the product is under APP.
your Plan (i) covers defects for the Apple-branded product(s) listed in your Planʼs Certificate or Proof of Coverage document (“Plan Confirmation”) and the accessories that are contained in the product(s) original packaging (“Covered Equipment”)
Also, even though there is no Apple 'branded' RAM (just Apple 'labeled' or authorized), if you upgrade the RAM (say, from a cheap third-party) and it fries your logic board (not very likely, but possible), it won't be covered. That's not to say if anything happens, you can't swap back in the original sticks that came in your computer. Or, if you upgrade the HDD, to replace it with the original if you take it in for service.
(iv) Problems caused by a device that is not the Covered Equipment, including equipment that is not
Apple-branded, whether or not purchased at the same time as the Covered Equipment;
Furthermore, Apple charges a ridiculous amount for their RAM, and they know it. Get it from a well known third-party (especially if you have the new Unibody).