As far as I know DEC's were never displacing SFO's who wanted upgrades. There were only so many SFO's eligible for command and any DEC's were "top ups". Certainly SFO's were told anyone who is eligible and passes the command assessments this year will get a command.
Last I heard was that if you joined the company you would be looked at during your first recurrent check and then could enter the process after that. It would be doubtful that you would get your command within 6 months of joining (unless they have exhausted the supply of internal SFO's eleigible for command). A year maybe more realistic, maybe longer.