You can NOT just update the shower PAN. You gut the entire shower, unless you are fond of leaks, and bad tile jobs.

The floor in a shower goes in FIRST. Then wall tile.

There is nothing really wrong with the pebble, actually better than the samples you are showing. Leave well enough a.l.o.n.e, unless you want a mess on your hands

Not familiar of total process involved to change just the floor (if whole space has to be redone) but consider using a shower pan maybe to settle down the look since room floor is dark.

This is disaster waiting to happen you cannot changew the shower floor without redoing the whole shower and what you have is better that waht ypou are planning . Maybe tell us a bit of why this change

Contractor said he could do it by grinding existing tile and laying new tile on top

And he will SIGN OFF on no leaks? I'd fire him now before you have the chance NOT to collect. All to get a selection not as good as what you have. Seriously?

Tiling over existing tile is acceptable as long as there are no existing issues with the tile or it’s existing installation. Many times it’s preferable to tearing out tile that is installed in a thickset mortar (though I doubt this one is). The one on the right looks better with the wall tile. I’d try to find a way to tie in the floor tile, though. Not sure those are your best choices.

The two samples are not the ones I am going with. I was coming to this forum for recommendations of what would look good.

You're bound and determined . A dark charcoal 2 inch mosaic unless you are changing the bath floor.