Thanks for highlighting this. Yes, there is a slight change in 4.2 involving the mavlink protocol used in that in 4.1 even if the user set SERIALx_PROTOCOL = 2 (mavlink2) AP would still wait until it received a MAVLink2 packet before switching to MAVLink2. With 4.2 it will “speak” mavlink2 right from the start.
As @geofrancis suggests though, it is easy to get back to MAVLink1. Just set SERIALx_PROTOCOL = 1.
… and just to be clear, there’s really only two protocols.