DENTI makes a very interesting point about the timing of your upgrade and and the state of your airline.
Had I got a command in the last year or so I would be in a far worse position than staying as an FO in my longterm base and near the top of the seniority list. Whilst I have missed out on the Command time, I would not have enough to get me a job as a DEC, I would probably be force moved out of base and take a 30k paycut. Not attractive.
We have a policy based on Last Promoted, first demoted, but it will be adjusted for sex equality issues. Some will also elect to stay in base and voluntarily demote, rather than be re-based.