I assume when you use the term " junior FO " this is someone new to the company?.
In the context of the current situation on the A320 fleet, I assume that's what GS-Alpha means, but it's also used in general for the FO's (and even SFOs) near the bottom of their status list. You can certainly be referred to as a "junior FO" for years if there's no movement on the fleet.
..and yes, normally you bid for type/seat change once a year.