Notwithstanding Typhoonpilot's comments:
If you have no union contract you're basically at the mercy of your management team.
God help you.
They make the rules and they can break the rules to their satisfaction. Sure, they've spent countless hours producing an employee handbook that outlines various scenarios but not every situation can be covered. And although it may seem like a binding document basically you have no recouse unless you'd like to pursue a lawsuit or something of that magnitude. In which case, then you'd be very lucky to ever upgrade.
The seniority system is just one way to manage pilots. Like Typhoonpilot, I once upgraded out of seniority much to the frustation of a few other FOs. In my opinion, the frustration was caused by the company claiming that on the one hand, yes, they do "respect" seniority and then on the other hand doing things like upgrading out of seniority.
In my opinion, if the companies want to hire low time pilots because they work cheaper then they should abandon the seniority system and stop pretending they "respect" seniority when they obviously don't.
And that's just one man's opinion. Mine. Good luck.