Hi,

Looking at the service manual the answer seems to be No you cannot upgrade without changing the systemboard.

On p.48 it describes how to remove the systemboard and there is no mention of removing the CPU, so it appears that it is permanently mounted onto the board

This is verified because on p.83-84 there are the part numbers (item #5) for all the different systemboards with different CPU's and onboard RAM configurations, available for your model laptop,