MAP sensors

Since MAP sensors assume the relation between air flow and air pressure is constant, would the ecu need to be retuned to accommodate for elevation changes or is that not related to this?

MAP sensors only measure the pressure. There are multiple ways an ECU can correct for changes in atmospheric (aka barometric) pressure (altitude), typically a barometric compensation table, which can be filled out as 0 bar = -100%, .5bar = -50%, 1 bar = 0%.