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.
Well it's not
that lich-like, is it. The Old Master doesn't have a phylactery-based ability to return to unlife after being destroyed, which is the lich's most useful power. Nor does it have a lich's fear aura, damage reduction, and immunities to cold, electricity, and polymorph.
Basically, the main similarity is it attack cause cold damage and incapacity, but a Ghast with sorcerer levels arguably has a more effective incapacitating attack than an Old Master - and those can be as low as 1st level characters.
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?
I'd rather just give it the skill points of its twelve Undead Levels and leave it at that. It just seems simpler, and we don't have to worry about what class it used to be before becoming an "Old Master".
The flavour text says they continue learning after becoming Old Masters, so the extra skill ranks an Undead gets over a sorcerer (due to one being 4 SPs/level and the other 2 SPs/level) could be due to its "undead experience".
I would add something like this:
Spells:
An old master has all the class features of a 6th level spellcaster. Most have the abilities of a sorcerer, but some old masters have the class features of another spellcasting class instead (a 6th level wizard, cleric, druid, et cetera). Old master's advance by class level, and can add these levels to their spellcasting powers. Thus, if the sample old master presented here advanced three levels in sorcerer it would have all the benefits of a 9th level sorcerer.
Skills: An old master's undead levels give it skill points equal to 15 × (2 + Int modifier). its undead level's class skills combine a neogi's racial skills (Appraise, Balance, Climb, Disable Device, Intimidate, Jump, Search, and Spot) plus the skills of whatever class it casts spells as, usually a sorcerer (which would add Bluff, Concentration, Craft, Knowledge (arcana), Profession, and Spellcraft to its undead class skills).