Do you have to wait for a p/t position to open to transfer shifts (preload to twilight)?
You need to check with HR what your local policy is and check with the Local Union to see if that's been addressed in any contract supplements, riders, addenda, letters of understanding, etc. For reference only, in my bldg (Metro Chicago, official HR policy is no transfer until after one year.
If so once a position is open do you have to go to the spot vacated or low seniority position?
Yes, you would have to wait for a position to open up, would be at the bottom of area seniority list. Might be possible to get on "preferred jobs list " (Article 22, Section 4 of the contract) for the other shift, I would recommend you check with your Steward or Business Rep/Agent