Just to the matter of attaching liner to a block wall...Just above expected water level, or higher, I drill holes into the block and secure threaded rod into the holes with either epoxy or anchor cement. Drill holes in 2x4 pressure treated wood for the threaded rod to come through. On what will be the liner side of the wood I counter sink the hole so the nut and rod won't stick out. Bolt the 2x4 to the block. Fill the counter sunk holes with silicone. The liner can be nailed to the wood with roofing nail.

The wood can also be glued to the block. A lot of DIY people seem to prefer glues and goops. They seem easier I guess. I find them more difficult. The other issue is how long it lasts. It's not the epoxy that is the weak point, its the wood delaminating.

You can also glue/goop the liner directly to the block. Also a pain and limited life.

When filled with water the water presses the liner against the wall so there's almost no downward pull on the liner. The water will keep the liner in place. Nailing the liner to the wood is just to keep it from flopping over. There should be no concern of the liner ripping away from the nails.

Sealing a liner to an old liner is possible if you know for certain the liner types, and if those materials can be chemically welded. It would never be my first choice. If you've never seamed liner together I wouldn't even consider this option imo. Hard enough with new clean liners that aren't inside a pond.

A couple of questions first. Is the new liner going along the back wall going to be the same length as the current liner? Is the current liner in good condition? Are there wrinkles in the old liner close to the wall? Are both liners EDPM rubber liners?
Mike

Thank both of you for your answers. Waterbug, it is not a block wall, it is an irregularly shaped stone wall, but I imagine we could drill holes into the rock for the threaded rod, I don't think glue would work. But how would we keep the liner from showing?
Mike, I believe the old liner is pvc, my new liner will be epdm. It is not folded or wrinkled along that wall, but it is brittle and not in good condition. The length of the entire pond will be smaller, but the part of the length that runs along the rock wall will be about the same.

Here are some pictures, you can see the rock wall on the left. Some of the stones are loose, some concreted:
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Sorry, I think I read "back wall" as "block wall". Little dyslexic.

For hiding liner in this type of case I make 'L' shaped fake rock that hang on the 2x4.
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Here's how I do it.

However, based on the photos you posted I would remove the rocks, lay the liner, and put the rocks back. When you said wall I pictured a vertical wall. What I see in the photos is more a bank. And I don't see any rocks large enough to drill and bolt to. Attachment is only needed if the wall is vertical or near vertical, so the liner doesn't flop over into the pond.

Are the rocks mortared? Is that why you don't want to remove them? It's easier than it may seem, at least imo.

Any pictures of the cement underneath or a good description would help too. Like is this a cement collar type deal or is the whole pond concrete under the liner.

The wall is pretty vertical. See pics below, the pond is now drained and much vegetation removed. Some of the rocks are mortared, some not. Some are very large, weighing a couple hundred lbs I would guess. So moving those is a challenge. Maybe it would be best to build a cinder block wall just in front of and below the rock wall, and wrap the liner around the top of that and hide with fake or real rocks like in your picture. Then I will only have to move the rocks where I want the waterfall, so I can get the liner under the rocks. Unfortunately, the best place for the
waterfall is right where it was previously, and that is where the giant
boulders are. Maybe a crowbar and some hefty manpower will work.
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The block wall idea sounds like a good plan since you're wanting a smaller, although deeper, pond.

Block walls can be pretty easy...I don't know if you have experience with block...I use all bond beam blocks. These make laying steel rebar in them easy. I just dry stack them and fill with concrete. No need to mortar the joints. You can make curves. I fill each course with concrete as I go so I can use a stiff (kind of dry) mix which I can pack into the block with a piece of wood. A dry stacked, curved wall will have some gaps, so the stiff mix doesn't run out the gaps.

The top course only needs to be filled a little and then you can tuck the liner inside. Or a pressure treated 2x4 can be anchored in the top course for nailing the liner.

You could get away without filling the block with concrete since they're be back filled, if your soil is pretty stable. Soil will stop them from falling back, water from falling forward. You can pack them with soil, and pack the back fill as you go too. No rebar. I would fill the top 1 1/2 course with concrete and rebar.

Thanks! Block wall it is!