the main concerns with upgrading prebuilts are space, the power supply, and proprietary form factors
a lot of prebuilt cases are extremely compact, so putting a new GPU in it can be an issue. if you have the case you can take measurements and compare them to the measurements of the card (available on the spec sheet of the website you are buying from).
for the power supply, a lot of times they use a very low wattage power supply, so to be able to power a new GPU, you would need to replace the PSU as well.
As for the proprietary form factor, I havent bought a prebuilt in awhile, but i know some of the older prebuilts had some proprietary parts. This can make it difficult/impossible to replace certain parts (like a PSU for example).
They say he's got a lot of chutzpah, he's really quite hhhhhip The parents pay the moyl and he gets to keep the tip