If you know a comfy set will help
Pay the money to upgrade.
Airlines take so much flak for the delays etc and have to pay compensation.
If the Airline presented a 'tech aircraft, with a damaged back" you would not board it. Just because you have a known defect with your back, should they board you? and upgrade you?
Pay the money for a fully refundable ticket, in case you cannot fly. Guess what, if you dont fly, they will refund you! Except you have had a go here!!!!!!!!!!!!!!!!!!!!!!!!!!!