Typically you can only be bumped out of your seat if your base reduces positions. And even then senior guys cannot bump you out. If you're 5 from the bottom and the reduction is 4 positions, you'll be safe. No new upgrades could come into that base and force you out.

Of course, we could write the language however we wish, but it would be seriously crappy to have senior guys bumping out junior captains just because they finally decided to upgrade. They can bid on new positions only. There has to be some benefit to biting the bullet and taking first upgrade.