Ryan works hard to keep some standard features across versions of the software. So you can flash the firmware back to something that resembles the original firmware. But you’d need to save a copy of the same version to be able to flash the exact same firmware.
If you aren’t interested in returning to the existing firmware, but you just want to switch back to milling, you can leave the printing firmware on it, and either just leave the extruder plugged in, or replace the thermistor with a resistor. But you can’t have dual endstops and an extruder without flashing the firmware.