Hi -- My shower is tiled top to bottom. The color/design/pattern is pretty outdated, so I'd like to take it all down and replace it. Big question -- if I pull the tile off and the cement board behind it looks pretty good, can I just smooth it and tile over it? Or do I have to replace the cement board?

Do you know for sure what is behind the tile?

Great question - I don't know what's behind the tile. I was hoping the cement board would have something stamped on it telling me it was.

If it can''t be 'smoothed' over you need to buy green board to place over top of it.

We do not want you to be topping what you have with greenboard, not in a wet area. Greenboard is definitely not for a wet area. I would suggest that you remove a tile and see what you have. Do not smash the tile with a hammer to get started, this will likely break what is behind it. Wear goggles and pry with a chisel at a grout line. The first tile is the difficult one but the others can be chiseled more easily with exposed edges.

All the contractors and home improvement guys in my area use green board. I thought it was the standard thing to use.
Is it maybe that they use the tub surround instead of tiles?

In a bathroom its OK but in a wet area you need something water proof not moisture resistant. Durarock, Wonderboard, Hardieboard or one of the water proof wall systems is what is called for. Greenboard Drywall - What is Greenboard Drywall

NoHelp, I do need to correct some of my wording regarding Hardieboard. Green board is moisture resistant but Hardie and the like are not water proof. They do not rot or swell and are rated for a wet area but the term water proof is slightly misleading. A moisture barrier is still needed behind the Hardieboard or Wonder board since water can pass right through them.