Sometimes the answer to a question is sitting right in front of one's face. I want a simple spell for 5e that transforms creatures from one form to another so I used Fabricate as the base.

 

 

Bioform X

4th-level Transmutation

Casting Time: 10 minutes

Range: 120 feet

Components: V, S

Duration: Instantaneous

 

This is a series of spells with each affecting only one monster type (so it is Bioform Plants, Bioform Monstrosities, Bioform Aberrations, Bioform Beasts, etc.). The spell permanently changes the form of a monster of no more than CR 2, Constitution save to negate. The resulting form must be of the same type, of the same or lower CR and of the same size or smaller. If the resulting shape normally has more hit points, it gains the current amount of the target creature (though healing can increase this to the new value). The resulting creature is not charmed by the caster but does retain any training it recieved before the transformation.

 

At Higher Levels: for every level above 4, the CR of the target creature increases by 1 (3 at level 5, 4 at level 6, etc.)

 

 

Which Bioform spells the DM allows is up to them. Personally I would only use the four types mentioned in the spell description (plants and beasts for druids, aberrations and monstrosities for wizards) and possibly allow Fabricate to modify constructs and undead. It all depends on the setting.