I already have existing animations that do not use IK bones. Now, for some new animations, I want to use IK bones. I have added IK bones to the armature, but when I go back to the existing animations, the constrained bones are all messed up (I kinda expected) because they are following newly added unanimated IK bone.

Is there a way to go back into each animation and automatically move the IK bones into position to restore the original poses?

For example, if I set the constraint influence to zero, the constrained bones will look correct, but I cannot do this for every action. Also, I am ultimately going to export it to FBX, and I don't know whether the influence animation will be respected.

I already have existing animations that do not use IK bones. Now, for some new animations, I want to use IK bones. I have added IK bones to the armature, but when I go back to the existing animations, the constrained bones are all messed up (I kinda expected) because they are following newly added unanimated IK bone.

Is there a way to go back into each animation and automatically move the IK bones into position to restore the original poses?

i.e. I would prefer if there is a way to take the rest pose, and the existing FK pose, then calculate the transform needed for the IK bones to deform the rest pose into the FK pose, then apply the transform to the IK bones.

For example, if I set the constraint influence to zero, the constrained bones will look correct, but I cannot do this for every action. Also, I am ultimately going to export it to FBX, and I don't know whether the influence animation will be respected.