There are subtle changes among CPU packages. I've yet to see such a configuration supported. Last time we wanted to upgrade a machine there was no way of getting the same chip as the one installed, and even changing the existing one and adding a twin wasn't possible as support for the motherboard was discontinued.
It is probably cheaper in the end if you just either get the same CPU (might have to rummage on ebay or such, can have a hefty premium), or move to a new/larger machine.
BTW, in Linux they are just starting work on supporting the ARM BIG.little (I believe 2 fast + 2 slow cores, can switch from one pair to the other) decently, all current systems assume the processors are identical (except for "hyperthreading" shenanigans, which are popular for a while now).