I…have no idea how to write lua.
but i hope/guess this is a simple request:
Im looking for a script that does a grand total of one thing:
change the entitie that uses the script into another, specifyable entity.
so if i for instance have stack_of_wood.json with a potion button that points at this scrip, it should be able to turn that stack_of_wood into stack_of_wood_on_fire.json for instance.
“Changing” an entity from one thing to another is not a simple (or even possible) task at all. What you’re looking for is destroying the entity (radiant.entities.destroy?) and then re-creating the new one (radiant.entities.create_entity I think)?
If possible, try thinking about using model variants and effects that are dynamically added/removed - unless there’s reasons that you want to have it a completely different entity.
Well the reason would be that I’d like to give the new entity a growth cycle, but I’d accept a “trigger your growth cycle to start now” as an alternative, if that’s simpler 
(destroy and recreate would indeed work)