Question: We will be installing a single under mount sink with garbage disposal. Converting from the current double basin sink.
I'd like to tackle this project if possible. My idea was to cap the right existing side, shorten the pipe coming out of the disposal and move that to the middle, where the new sink hole will be. I am also planning on purchasing new garbage disposal, but that should not affect anything.

Hoping everything will align. Does anyone see an issue with this approach ? Attaching the existing photo and also marked up photo of what I'm proposing to do.

[![enter image description here](https://i.stack.imgur.com/K0nao.jpg)](https://i.stack.imgur.com/K0nao.jpg)

[![enter image description here](https://i.stack.imgur.com/42FSS.jpg)](https://i.stack.imgur.com/42FSS.jpg)

Answer: 
[![enter image description here](https://i.stack.imgur.com/TQ4rb.jpg)](https://i.stack.imgur.com/TQ4rb.jpg)

Your dishwasher will connect to the port on your disposal unit (green arrow) and the drainpipe/crosspiece from the double sink (marked with red "x") will be discarded.

The tailpiece from the disposal ("A") will be replaced with one that has a 90-degree bend. (This piece will likely be included with the new disposal or could be purchased separately.) This piece will connect directly to the drain trap.

The part that might be tricky is getting the disposal tailpiece to line up with the trap. More accurately, you'll need to get the trap over to where the disposal tailpiece is. To do this you will need to rotate the trap and or disposal to make things line up.

This might require adjusting the trap extension at "C" or, in an extreme case, modifying the plumbing at "B" to bring the trap assembly closer to the disposal, but without measurements, etc. I can't tell from the photo.

What you're looking for is something more like this:

[![enter image description here](https://i.stack.imgur.com/R5xCC.png)](https://i.stack.imgur.com/R5xCC.png)

(This is the best I could find to steal... Notice the exit from the disposal connected directly to the trap and the dishwasher drain connection to the disposal.)
Don't cap. Don't cut.

Remove everything before the trap. The new disposal will come with an L-shaped tail piece that comes out then down. The new disposal and the existing trap can both be rotated. Use the new tail piece and rotate them both. I'm pretty sure they will align. If not, try shortening (cutting) the 90 degree bend coming OUT of the trap, or else get a tail piece extender.

[![enter image description here](https://i.stack.imgur.com/MTurm.png)](https://i.stack.imgur.com/MTurm.png)

That AAV should be higher. Ideally higher than the highest water level in the sink. But lacking that, at least higher than the drain. You have to hope the new disposal's exit is no higher than the existing one. If it is, you'll have to raise the AAV. That's glued in so you'll need to cut it all out and do the whole thing from scratch.