Upgrade:
Upgrade one unit of Crossbowmen to be replaced by Handgunners. Only one Unit of Crossbowmen can be upgraded to Handgunners regardless of the size of the army.
Is this some kind of Power Keep?
No since you can still decide if you prefer the sword of might over this upgrade. It is not going to be a must have choice.
It is simply more variety in smaller games and has little impact on larger battles.
The Points are nothing new and well tested and selected.
Does other armies want a similar treat?
It is just playing with the Min/max without imballacnng the concept as a hole. especially since it is not a new Troop type. I don't see many armies coming along able for the same kind of swap, do you?