Mk2s to mk2.5s should do you Jesse...
The mk2 to Mk2s Upgrade was quite minor, Mainly new smooth rods, bearings and Ubolts for the Y bearings.
I didn't bother with it.
Both Mk2 and Mk2s used the old small single sided hobbed gear for filament drive.
Mk2.5 and Mk2.5S both use the |double sided, larger diameter, Bondtech gears. so you can miss the Mk2.5 extruder stage, and go straight to the Mk 2.5S extruder
the Mk2.5S extruder is shorter than the Mk2.5 extruder and the mk2.5S uses the newer filament sensor. so potentially in the future the firmware will allow us to use the extra height