Remember, though, a great old master is a giant, bloated thing, but the undead old master is still like a regular adult neogi. So I don't think the great old master makes a good comparison for the undead old master.
Given this quote, "They move slowly, but in spell use and anticipating enemies, they are very, very, quick," I think I'd rather decrease Dex slightly than decrease NA, but only to Dex 18. Speaking of speed, does speed 3 really translate to 30 ft? I thought 30 ft was a higher movement rate.
I'm starting to think we should bump up the casting ability, though.
OK, you propose Speed 20 ft, climb 10 ft and Dex 16? I could go for that.
The original critter says "Most encountered undead old masters are the equivalent of 8th-level wizards (to find an individual’s level of spell use, roll 1d8+4)." The CL 6 is a holdover from the LoM neogi sorc, right? So I'd rather bump the CL to 8. That would help explain how it can become a "lich-lite," too.
So I'm fine with it as a 6th level caster like at present.
The main thing is just that 6th CL seems a little low for gaining some lich-like powers. Think of it as a template prereq. Also, a boost over the "typical" LoM neogi sorc seems reasonable. But it's not a big deal.
Along those lines, you raise the issue of skill ranks. Do you want to make these "template-like" so they have the ranks of their casting class?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.