I been seening games like Tales of Mystra who has sprite growth for their things. As for growth, I imagine the best way to implement it would be an action, which may be auto cast or manual casted to replace the sprite with another multi tile sprite. Of course that would mean multi tiled sprites must be done first, and it would be interesting to see a multi tiled boss happen.
As for conversions, the best idea I had, is if you can read and copy levels and maybe more of a character and copy them onto a new character, mechanically you wipe or destroy the first character you want converted, then create a summon, as you are programming now, with the levels effects and such matching that of the destroyed character. Mechanically, it's two separate entities, but narratively, it'd be one and the same.
In converts where you are trying to tame, take the unit type and transfer that over too. They are TECHNICALLY different people, just a summon being created at the same time the old character is erased, but it should work for what your trying to do.