Ok ... here goes.
Apparently the only thing you cannot do with the units in the rules editor is
rename them.
This is apparently due to the huge pile of files that still remain with the wrong name, in the Art\Units\<unit> folder... including the FLC files.
The FLC files are frame-by-frame movement of each unit, but not the actual statistics of that unit. However, these are necessary to animate the unit, and while they are given under the wrong name, the game engine does not know how to find them... which explains why the game doesn't crash until the renamed unit in question is produced (thus it then needs to be animated...)
Any type of modification can be made on the unit's information and statistics in the editor, like traits (I.E. move all spaces as road), prerequisites (such as Iron Working), attack & defense, bombard, the ability to bombard, etc etc..
If you don't change the name of the unit, you can do practically anything to it as long as you don't try to make something ridiculous that the game doesn't know how to handle (such as a knight that can walk on water ... I tried that ... hahahaha.)
Here is how to do it: (Holy Knights being my example, for my new Fundamentalist government... and no, it's not a super-cheese kill-everything like fanatics were in Civ2):
If you create a folder in the Art\Units directory named after the new unit name you want... (I.E. Art\Units\Holy Knight)
Then make copies of all the files inside the directory of the original unit you're renaming (I.E. Knight)
Paste those files into the Holy Knight directory and change all the instances of Knight to Holy Knight.
Open the Knight.ini (which you should have renamed to Holy Knight.ini!
Change all instances of Knight to Holy Knight.
This theory has been tested now... and works.
So have fun! :crazyeyes