I have a Psion(Egoist)/Thrallherd with Psychic Reformation. I'm wondering whether I can use Psychic Reformation to drop the points in Diplomacy and the Inquisitor feat, and reassign the points to something else. Also, when I reform myself, I remain a Thrallherd. Does this mean I don't have to spend 8 points on Diplomacy, as it's already a class skill, thus letting me just put the required 4 points into Diplomacy, and use the other 4 on something else?
Leaving aside the question of whether any self-respecting DM would let you pull this kind of crap...
As per the SRD, "the subject can choose to spend its most recently gained skill points differently". To me, that means that you get to spend those points within the same constraints as when they were originally used, so you wouldn't get to count Diplomacy as a class skill.
And if you're planning to get them in a new level, you're forgetting that once you no longer meet the prerequisites to enter a PrC, you can no longer gain a level on that PrC until you meet them again (and, depending on whose word you take, you also lose all benefits of the PrC). Therefore, as soon as you level up, you won't be able to gain a new level of Thrallherd.
I really feel like this use of Psychic Reformation goes against the intent of the power. The power is intended to let you remake poor decisions, not eliminate prestige class prerequisites. As a DM, I would never allow a player to Psychic Reformation away their prestige class prereqs.
That said, the rules text backing that up is very shaky. There's no mention of prerequisites in the test of the power, and there is no errata mentioning them either. The only thing that's related that I can find in the text of the power is:
This makes me believe that you cannot end up with an invalid character after you finish redoing skills and feats. Since you can't take Thrallherd without first taking its prerequisites, you can't Reform those prerequisites away.
Again, this is not a strict rules interpretation, it's more of an inference about what the intent of the power is, and a logical extension of the power's rules. In any case, if you no longer meet the prerequisites for Thrallherd, you can't take further levels of the class.
Sounds good to me. Neither the skill points nor the inquisitor feat are required for any of the thrallherd's abilities to function, so losing the feat/skills wouldn't de-power it's abilities, you simply couldn't gain further levels in the class till you reacquired the pre-reqs. Though, if you're only going for 1 level, it might be worth more to just pick up the leadership feat. Though, after reading through power documentation, it seems fully possible, if you're willing to spend the xp on it, to take 10 levels of thrallherd, then Reformat yourself back 10 levels and reapply all those skills and feats however you want. Sure your DM can rule however they want, since their word trumps source material, but by the wording itself this is perfectly acceptable.
And no where in the description does it say you must spend the skills based on your character at the time. So yes, you technically could take the 8 points out of the 4 cross class, and respend them as a class skill which it now is.
If anything, the only problem I see is that the power level seems a bit low for what it can do. The only arcane or divine way of replicating this effect is through wish or miracle and since it's not listed as a safe wish, the DM will probably screw you for trying it. Maybe if the power was 8 or 9 and/or the xp cost was 50 xp per character level per level you wanted to revision, it might be a little more balanced.
If you no longer meet the pre-requisites for anything - feat, prestige class, spell, skill trick, magic item - anything at all - you lose the benefit of that thing until you once again meet the pre-requisites.
I am aghast that all these answers and no-one has mentioned this.
For a prestige class, you retain the hitpoints, skillpoints, BAB, and saving throws but you lose any special abilities, any spell advancement, and may no longer take additional levels in the class until you once again meet the pre-requisites.
Since this makes the option moot, unless you were in it for that stellar Thrallherd BAB, I believe that's Question Answered.
Thank you for not making this rules-as-written because
Opinions Abound, but Absolute Answers Are Absent
In every errata file there's the Errata Rule, which says
The Dungeon Master's Guide--the primary source for information about meeting prestige class requirements--says
Complete Warrior, the FAQ, and Complete Arcane
Complete Warrior says
The FAQ cites the Complete Warrior when answering the question
Complete Arcane says
Complete Warrior and Complete Arcane are the only texts removing prestige class features upon failing to meet prestige class requirements. And while language identical to that in Complete Arcane was present in the Dungeon Master's Guide (2000), it's absent from the Dungeon Master's Guide (2003) and the Dungeon Master's Guide (2013).1
Examples in Play
An Unpleasant Compromise
The strictest reading is that the prestige classes in Complete Arcane and Complete Warrior follow the rules given in those books for losing prestige class requirements,4 but prestige classes from other sources don't.
Talk to the DM
I suggest the DM make clear his house rules about prestige class requirements before play begins.