Apple will not replace a computer under warranty with a newer model, first off, and in general they will do several things
1) pretty much replace any damaged part, from logic board, to cpu, screen etc, including body, to get the machine to work, and trust me, they have parts on hand going back five years, at least(no written evidence, but i have experienced it recently)
2) If they deemed your computer completely dead due to a manufacturing defect, say a year or two later, they would either do 1), or as most would assume, say that the damage was user caused, now Apple does not usually(almost never with MBP's) blame the customer, so they would then do 1)
3) in the unlikely event that they decide on a replacement, highly unlikely, they would most likely give you the same model refurbished, i am confident that they keep models in backstock for three years from release date for this very purpose,