I think it should be a neat mechanic of letting us use hearth magic to upgrade already built contructions either those that produce items or just constructions in general.
In theory I think it would be like have a piece of item of higher quality for said construction that has been already built, let's say Kiln that was built from quality 1 clay because nooby didn't know any better, so after a while leveling up masonry to 10 and now the nooby has quality 10 clay because kiln is made from clay using hearth magic let's say whatever they decide to call it used something like 10K XP and consumes those xp and that quality 10 clay turning that kiln from quality 1 to quality 10. And oh yea it has to be all of items needed for said construction to upgrade it.
I am not sure if they decide to make the xp cost fixed or variable. But would speed up a process instead of having to build new constructions we could just upgrade them with Hearth Magic. And hearth magic would have more use in game.
But the middle part was just an example of how it could be maybe done, I'll let you lot and devs to deicide how it could be done better.