Oooh yeah... Horizontal Movement isn't easier when you take that into account, is it?... It might be just as impossible. But you could make an item with the ability to be changed... Eg: Goes 100 units to the side and back, textures GFZROCK, ceiling: FLOOR0_6, floor: FLOOR0_3. Then have the ability to change these to different textures.
Yes, this is impossible and I'm just wasting your time. (YAY!) Hmm... I know! A flipping tutorial on how to add enemies and flats! (already knows textures and flats... but the flats either look glitchy or have the wrong colors...)... By adding, I do not mean movement. I mean add. Just plain add. Ohnoes, teh bye!