They can change map with a switch, so it would be hard to do it while changing gears. But I do know they have a special map for the starts.
They change maps like they change gears - by hitting paddles. One shifts gears, one shifts maps. On the Mclaren, when it was first introduced, right after the TC ban - the shifting paddles were physically arranged in such a way that activating them activated the map paddle simultaneously. Not very difficult even to activate the two paddles at the same time with just manual control.
But we KNOW that Mclaren runs this system, so Renault would have to have developed it to a muuuuch higher level of refinement in order for it to create such a big delta on starts.