As far as I know, seniority has no bearing on upgrades. That's on hours and merit. (as it's always been).
Fleet changes is now seniority influenced but if I understand the mix correctly, it's based on date of joining as well as what fleet you're on and a few other things. So, it's fairer and a lot more transparent.
Let's see if I have it correct. This is an example.
Mike, Bravo pilot, joined Feb 2006. (small cabin) Number 301.
Dave, Excel pilot, joined Feb 2006. (medium cabin) Number 299
Steve, 400 pilot, joined May 2004. (small cabin) Number 10
All now captains. (seniority played no part - just merit and hours)
Mike just wants to fly the G550.
Dave things long haul is death and wouldn't mind the Hawker 4000 or the Falcon 2000EX.
Steve, he just wants the 900EX.
With this simplistic example, unless there's a delivery of a Falcon 900EX soon, Steve (*10) will fleet change a long time AFTER the other two guys. Why? The 900 EX is a very small fleet.
Dave (*299) has a "fleet lock" of 3 years versus 2 years for the small cabin guys. Therefore, he can't change fleets before Mike who joined at the same time.
Now, if Mike realises that the G550 has many people with a "better" seniority number than him bidding for it, he can change his selection to "Falcon 2000EX". Since he's out of "fleet lock" earlier than Dave, he can bid for it. (the bidding system will show how many are "waiting" etc)
The first part of the new list has been implemented, now just waiting for the rest of it.
Please understand that this is my interpretation of the new system so it may be wrong and it is a very simplistic view of it. (I am just a line pilot)