Transitioning to other aircraft, like EVERYTHING at the airlines is based on seniority. Your seniority starts the day you’re hired and continues to accrue after that. If I am hired the day before you, regardless of what aircraft I fly I will ALWAYS be senior to you. Periodically due to changes in staffing needs (new aircraft, new routes, retirements etc) the airline will have a vacant pilot slots and will open a “vacancy bid”. These bids can happen a few times a year or not at all a year. Again it depends on need. Any pilot can bid any slot they want on any aircraft, any seat (CA or FO), at any base. Whether you get that slot is based solely on seniority and NOTHING else (there is no approval). If we’re both 737 ORD FOs, both bid ORD 767 FO but there’s only 1 ORD 767 FO slot and I’m senior to you, I get the slot and you don’t. End of story. There may be no ORD 767 slots but there may be a SFO 767 FO slot. Maybe I live in ORD and don’t want to commute but you don’t care and simply want the plane. If you bid it and I don’t you would get it even though I’m senior because I didn’t bid it. Now once I’m awarded that slot there will be an effective date of the bid and the company needs to train me by that date (usually 6 mos) or they must pay me for that aircraft whether I’m on it or not (bypass pay). You’re trained for that position in seniority order so. If there are only a few transitions it will usually happen fast, if there are many it could take a while.
Now again your “system” or company seniority never changes BUT your “relative” seniority within a certain airplane or base can and does. There are junior airplanes (usually based on size/pay) and junior bases. When I was at ExpressJet I was based in EWR which was a Junior base. I gained “relative” seniority at that base very quickly because many pilots wanted and did move to other bases when they could but I stayed. Right now at Hawaiian we only have one base BUT I’m currently a mid-seniority A330 FO. I’m going to be a very junior 717 CA once I get trained but I could be a very senior 717 FO if I chose to be. It’s really a matter of personal priority what you chose. There are many pilots who could hold A330 CA but stay 717 CA because it offers a better quality of life vs pay.