Well when you trade things in at gamestop they give you money.. correct? so i couldn't see why you couldn't get a new PSP. you may need to pay a little extra though.
it it probably possible. i did it once and it worked. but mine was in mint condition but it didn't have the Star Wars Renegade squadron. so i don't know if it will work if it is not in mint.
i don't think you can if you got a used one you can return it in a minimum amount of days. and if you wanted to get a newer one you would have to buy it... because i don't think they would rather have a used one then a new one.
good luck! i could be wrong but that would be my guess...