The pump impeller can be accessed by using an adjustable slip-nut wrench to turn the disk mount assembly counterclockwise. Once the disk mount assembly is removed, the impeller can be removed.

Very often the disk mount is hard to unscrew because of minerals build up in the tread. In such situation it has to be broken in pieces. You can use a chisel or a flat blade screwdriver. Be careful not to damage the pump body.

- The disk mount Part number: AP2909490

okay, got this one figured out with a little bit more research, actually on this site. If you find yourself in the same predicament that I did, you can actually break apart the impeller with chisels. I felt a little bad using my decent woodworking chisels to do it (I couldn't find any of my others - perhaps time to reorganize the shop?), and I will have to resharpen my 1/4"er, but it works. Took a few whacks with the mallet and chisel and I was soon pulling the remains of the impeller out.
Because of the rust that had locked my impeller to the drive shaft, I spent some time sanding and cleaning the shaft. Not a machinist quality of clean, but it went back together with some gentle persuasion, and liberal application of white lithium grease. It's running a test load now, and NO LEAKS!
Pretty impressed that an 18-19 year old dish washer still has easy parts availability, and is still running. total parts? (including a fresh water supply line) $43.32