EXACTLY which switch? Most likely the answer is no. At least for some of the switches, the 2.x firmware goes with newer hardware and is not compatible. For example, I have two RB260GS switches. One is the "classic" RB260GS and the other is the newer CSS106-5G-1S. They look the same until you read the fine print on the label. The classic switch takes 1.x firmware and the new one takes 2.x firmware.
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
Not sure if I beat them in submission, or they beat me into submission