You are suggesting that CX will do away with seniority?
Will these DEFO's pax be on a 'D' scale?
Has this been run by the union? Don't think it has much chance of getting through.
Plenty of CX S/O's with plenty of experience to be FO's and have been in CX system for a while now, think the company would/should consider them first.
If they are keen on doing DEFO pax as you say, then just do away with S/O's and employ all new joiners as F/O's. They would probably do it on a lower payscale for first 2 years (maybe 2nd year S/O pay)