I know this is a simple question. Forgive me. I have old flagstone where I want to plant a field. Can I remove it? Replace it with soil? How to do that?
Use the remove floor command (either in floors or orders tab) on the old flagstone and the old soil below it should appear where you can setup a growing zone again.
Don't be alarmed when you do not receive materials back. Flagstone is intended to not return materials on deconstruction. Unlike tile floors, which do return a portion.