Firstly, my PZ3 is here: http://www.magotek-industries.com/Files/sheath_back_sword_preparentchange.pz3

I need some help with this. My aim is just to get two static poses that I can import back into the main file. I'm animating a sword, specifically a sheathing animation. SO the utmost important thing is to get the sword where I want it, and the arm has to just accomodate that.  Just rotating parts of the arm really isn't going to work well here. So I'm trying to parent the hand to the sword, and translate the sword around, having the arm automatically adjust itself to keep ahold of the hilt.

Where are things going wrong in the following example:

1. TO start with, the sword is parented to the hand, and in a position that is almost where I want it. I make a backup save file that will not be overwritten here. This is the file at the top of this post.

2. I go to frame 25, which is closest to the pose I want. I select the sword, and set a keyframe for it. This is necessary to prevent it from moving in the next step.

3. I unparent the sword, by changing it's parent to Universe. Without the previous step, the sword would now move somewhere odd. I've found that with the keyframe set, it stays in place. The sword and hand are now not linked in any way, I think.

4. I select the right hand, and change parent to the sword. This causes a slight distortion to the wrist which iis due to the sword having been scaled down a little. I think this is irrelevant though.

5. At this point, I save the pz3 in a seperate file, then test the bvh export. It works perfectly. I reload the pz3 to get rid of the extraneous keyframes.

6. I select the sword, and translate it to where I want it to go. Basically just down a little, so that the blade is touching the top of the sheath. The whole right arm moves realistically to accomodate it, exactly as intended.

7. I attempt to save the current pose to the library, as it's one of the ones I want.. At this point, my figure's neck stretches stupidly long, the left arm goes into a straight out pose, and the sheath on the back falls to the floor. What the hell is going on ?

8. I make a reasonable assumption that the parenting has something to do with this. I reload the pz3 from step 5, and I change the parenting back. Firsly, I change parent the right hand to right forearm. This causes the figure's tight hand to remain in place, but the forearm and shoulder return to what they were at the start, resulting in a stupid looking distortion in the wrist.

In any case, I can't seem to get a useable pose out of this. Please help. Someone in another thread suggested deleting IK chains might help, but I'm not sure how to do so. The Heirarchy window only has options for creating chains, not deleting them. and the button is greyed out.

The first thing:  You can't change parenting in the middle of a PZ3.
This is one of Poser's biggest 'gotchas' in animation.

One workaround is to 'stop' the PZ3 at the frame where you need to
change the parenting structure, then start a new PZ3 from that point,
with the parenting setup changed.

Then shouldn't changing the parenting change it for the whole thing?

Or will I need to save poses and reimport them into a new pz3 ?

1. I have no experience with reading pz3 files and cannot open your scene without your character props and textures. Refering to real life I think that sword should be parented to hand ;)
   As was metioned above you can parent object to object for whole scenes. Parents cannot be changed during animation.

Quote -

1. I attempt to save the current pose to the library, as it's one of the ones I want.. At this point, my figure's neck stretches stupidly long, the left arm goes into a straight out pose, and the sheath on the back falls to the floor. What the hell is going on ?

Your pose cannot be saved correct, because some body parts are animated refering to prop. For example transfering X unparented hand you move it along body X ax. Parented hand moves along prop ax - if you rotate prop along Y ax X ax is completly different.
Ref neck behavior

• you could generate some bug. Sometimes, when I calculate cloth dynamic, head of figure explodes or hairs fly away :D It happend in complicated scenes.
• you can set IK for head or neck, when you changed IK chains.
  Open again your scene and check what body parts you checked saving pose.

1. If you want to change parents during scene you can use 2 swords (with dummy props). Check prop/sword properties - "visible" option can be animated. Animate swords together setting 2nd sword unvisible, when you are in desired frame, partent hand to 2nd sword, make unvisible 1st sword and 2nd visible.

Anyway you are trying to make something very complicated. Remember that you can make 2 scenes and later join avi files in 1 movie.

I'm not making a movie, which will perhaps remove some complexity. I'm creating bvh animations for second life. All I really need from this is two single frame poses, so getting them from another pz3 is sounding like the best idea.

If I were to use two swords, would the hand move properly when the other parts of the body move? eg, if the hand is parented to a sword, and a second sword is parented to the hand. And wouldn't doing that cause the first sword to be left behind when I move any part other than it ?

I'm not sure if I understand clear.
I imagine your animation:
*Figure moves/fencing right hand parented to the sword (hands is translated along the sword moves). In one moment for example frame X figure is taking sword to left hand and fence with sword in left hand.

*-Make your animation from 0 to X frame with right hand parented to sword 1.
-At frame X pose left hand holding sword.
-select sword 1 on properties window, click copy (ctrlC) and make sword 1 unvisible
-seclect sword 2 make it visible, on properties window, paste (ctrlV) and parent left hand to sword 2
-Animate sword 2 in next frames. Left hand should go follow it. Of course forearm, arm etc should move follow.

When you finish your animation turn off IK. Check animation again what is broken. Most often you must change 1st frame only changing twist or bend or side-side parameters adding/removing 360. For example if twist param=210 change it for -150 (210 - 360 = -150). Hand should stay in the same position.
When your animation work correct with IK off it is ready to be saved or exported.

The sword will only be held in the right hand. Left is irrelevant for this.

Basically, what I'm worried about is that with theright hand parented to one sword, it won't be parented to the right forearm . does this mean that if I adjust the arm, the hand won't follow, and will just stay hovering where it is in space?

Hand can be parented only when you turn on IK. So when you off IK hand will follow forearm.
Moreover you must turn off IK before you save bvh or pose. Another way bvh or pose will have false parameters - hand position related to not existing prop.

Alright, this still isn't working. I tried the following.

Firstly I started a new pz3 with the following setup

Two swords:

Sword A is the parent OF the right hand
Sword B is parented TO the right hand

I translate sword A around to pose.

When I'm done, I turn off IK as instructed. And this happens:

The right hand baloons up horribly. The swords are at 66% of their original size, which may have something to do with that.

So I tried saving the pose anyway. This is with IK OFF. Just adding it to the pose library on the right. It seems to save just fine.

Then I tried reimporting it into my main file. Just a static, single frame pose. And it came out like this.

What am I doing wrong ?

I've made few scenes trying to obtain effect similar to yours, but in my animations all is ok.
Check PM. I've sent you small sample.

I have no more idea what did you do wrong.