Your shower tile is surface mount so you can simply remove the trim piece at the point you want to start the subway tile and butt it into the existing square tiles.
Adding tile to existing tile.
Hold the tile close to your device until they connect it should take about ten seconds.
Sounds like this is a new adventure for you.
Tap the on the top left corner of your screen.
Grind them down with a grinder.
It had the same rustic feel.
It was 13 x 13 the exact same size as the old tile.
Tile loads are usually higher than 10 psf so chances are your trusses were not designed to account for this.
It was relatively solid looking.
Roughen the tile with 80 grit sandpaper.
Layer newspaper on the floor across the bottom of the tiled wall.
It did not look like we had tried to match the existing tile and failed.
Lay a level on the old tile and mark any high spots.
By adding cement board and tile to the floor you will be increasing the top chord dead load.
Open the tile app on your smartphone or tablet.
On the add a device page choose tile for tile partner products follow the prompts on your device press the button on your tile device and listen for the tune.
If your old ceramic tile is worn or dated you can lay new tile right over the old and avoid the huge job of tearing out the old tile.
Then rinse the wall with a water dampened.
A couple other considerations are the on center o c spacing of the trusses.
Scrub the wall tiles with diluted degreasing cleaner and a nylon scrub brush.