A dwarf died, and the room I wanted to put the coffin hadn't been mined out, and In a rush to get the corpse safely stowed away, I plonked the coffin in the hallway outside where his tomb was to be. I want to move it inside, but I don't want to click "Remove Building" for fear of it just being gotten rid of. Can I, for instance, safely select "remove building", whereupon it will be put away into the appropriate stockpile and I can place it again. With the same person in it.
You can't move a coffin and a dwarf together, but you can move them.
A constructed coffin is essential a building containing two objects: the coffin item itself and the dwarf. If you deconstruct the coffin building, both the coffin item and the dwarf corpse will be placed on the floor, then moved to stockpiles/available coffins by dwarves as appropriate and available.
You won't lose any of the parts by deconstructing it, so as long as dwarves are available and you manage what they do you can effectively move the whole thing.