Replacing the lower half of a fully tiled bathroom

First thing to check is that you can get the same tiles as the top half (unless you want different tiles for the lower half). You need to take care removing the row of tiles next to the ones you want to keep (I imagine this is the border). Use a grout rake (available in many hardware shops) around these tiles to disconnect any potential shock to other tiles from the hammer or chisel. Drilling 4 holes close to each corner of the tile can also help to avoid others cracking further along the wall. Then use a hammer/chisel carefully to remove these. Obviously you can be less careful with the rest underneath once this row has been removed. You're correct, tiling downwards can be tricky, but you have 2 options. Knock a nail, or screw underneath after laying each tile to hold it in place (remove when ready to grout), or dry lay a tile underneath the border, then draw a pencil line around it. Continue this process downwards until you reach a point where you can screw a spirit level into the wall and use this as your starting point. Remember to factor in 2mm for spacers when drawing templates on the wall. Don't forget to plan for what might appear underneath the old tiles. If it's an old property and the plaster crumbles you may need a plasterer or have to use cement boards to ensure you have an even surface to tile on. Hope this was helpful.

This is possible but can be awkward. Bear in mind if you are matching existing tiles there will be a slight shading difference.