No - anytime you're under duress (read: torture) of some sort, I'd say you need to make a Concentration check to maintain your disguise. I'd use the same method as spellcasters who take damage when trying to cast a spell, 10 + damage dealt DC. As far as the knocked out...hmmm...I'm not sure...it would depend on the situation.
I am sure that a changeling knocked unconscious or killed would revert to its true form. What if he or she were subjected to torture by someone who suspects the changeling's true nature? Would a will or fort save against x damage inflicted be fair?
How would you handle similar situations? Would you just let the changeling keep up the disguise no matter what, in order to keep thing simple?
Torture in itself shouldn't be enough to trigger a reversion to natural form - if that was possible, then it would also be possible to force a change through regular combat or other pain inflicted by traps, etc. (the debate would center on what threshold there was).
Of course, as DM of your game, you have the right to establish such a house rule, but that weakness would dissuade players from running shape-shifting characters. Personally, I would allow a Concentration check as Lilith, or a Will save, or both - only if the shape-shifter is using a supernatural ability and not a spell-based one, since a spell ability that doesn't require concentration will not be swayed by torture, regardless of the pain and damage to the caster.
I personally would have a shapeshifter hold form if unconscious (how inconvenient it would be to sleep in changed form!) but not if dead (for obvious reasons).
Of course, if there is suspicion of a changeling, there are other means...
A high enough level Mage can cast True Seeing, as well as Dispel Magic (in the event of spell-induced Polymorph or Alter Self as opposed to supernatural changeling ability) or Arcane Sight to detect magical alteration.
M
A changeling's disguise is basically just a fire-and-forget magical effect. He activates it once and afterward it needs no concentration. Here's the list of the only things that will disable it:
He might remove his disguise voluntarily if coerced or threatened into doing so. He'd give up his true appearance in the same manner as he'd give up any other secret.
Oh, then I suggest the use of a whoopie cushion. A well timed whoopie cushion is like kryptonite to the Real Life changelings.
I would disagree that dispell magic or an anti-magic field would drop a changeling's disguise. Other than that, it would be death or changing to another form. Also, true seeing can see through it. Also, I like the concentration / will save. I'll have to try that.
Q - What would cause a changeling to revert to true form?
A - You get him elected as president!
Ouch!
Ultradan