A small number of properties for a unit can be changed via modify protounit trigger and some techs but otherwise unit modifications are not possible.
It would be nice to expand to provide full API for editing all unit properties listed in there (Other than critical ones such as object name, unit type) which would include:
* Adding/Modifying/Removing properties such as obstruction radius, bounty factor, turn rate, decay time.
* Adding or removing the many supported flags.
* Adding/Modifying/Removing action types. E.g. Change the attack on some special attacks, or the radius or splash attacks, Add or remove regen persistent actions, even silly things like have minotaurs gore units across the map etc.
* Other property changes such as changing what units can be trained, changing what techs can be researched, allowing other units to carry relics, etc.
The possibilities this would provide for scenario designers for custom games is enormous!
Maybe the simplest way to enable it would be to allow an xml fragment to be accepted for a unit id/name and it overrides the protounit data with the provided fragment but there are probably neater ways.