OP - It definitely sucks and I’ve had this happen myself. I’m a Diamond and they still wouldn’t do anything. But it sounds like staying on your original flights is important so I would first grab the one open seat there is and then either:

A) Create a seat alert on ExpertFlyer to get notification of when another D1 seat opens so you can grab it

or

B) Check the seat map routinely 1-2x a day over the next 30-days, specifically ~7 days till departure. Peoples plans change and 1 more seat will likely open.