I built a couple of workshops on the clay floor. Right now I am redesigning a little to have marble wall and floor and the clay color underneath the workshops bothers me.
Is there any way to change the floor underneath a workshop from clay to marble?
I'm not sure what exactly the other answer is referring to with 'Replace Floor', but you can achieve your desired result by deconstructing the building and changing the floor tiles to be the same as the surrounding tiles, either via smoothing d > s (assuming the underlying tiles are marble) or building a floor b > C > f and then selecting marble.
I'm relatively sure Replace Floor still works under a workshop. Even if it doesn't, assuming the workshop isn't one with an active lava channel under it (if it is, that clay will probably not hold up for long), you can just dismantle the workshop and replace the floor and rebuild the workshop without losing any materials. If you can't live without the workshop for that period of time, build another one first, then tear down the old one.
