>>384541949
So if you ever use the pose mode to move the armature, you need to select the "Apply as rest pose" option to lock that new pose in as rest pose, otherwise it'll go back to the original pose for exporting.
But if you go set the new pose as the rest pose, the meshes that were deformed along with the armature will go back to their original shape, since the meshes were using the movement of the armature as the modifier to deform from their original shape.
So before applying as rest pose, you need to go to the individual mesh object(s), to the armature modifier, and save the new shape as a shapekey. Once that's done, you can go back to the armature to set the new rest pose. Then go back to the mesh object and use the shapekey to propagate the deformation to the basis.
That's how you set the new default rest pose for both the armature and the mesh.