it will all happen at once
The main insight I have from the LG source code is that LG has things nicely structured so that there can be parts that are:
A) unmodified from the official Android-MSM7xxx branch
B) changed for all LG hardware
C) changed just for specific families or groups of LG devices
D) changed just for our LS670/thunderc/whatever
It works in a top-down way. So, when they upgrade to gingerbread, they will do it in a sane way, which means they will upgrade a whole bunch of phones (probably, all of the models they plan on ever upgrading, and including yet-unreleased models) simultaneously. It is the only dev process that makes sense.
They have to change all of the above layers. First, base off a new official branch and different version of the Android platform. Then, reapply their global LG patches. Then, reapply their device family patches. Then, reapply all of the per-device patches. Then, run their tests on all the devices. When there's a bug found, fix it at the appropriate level, remerge everything downstream of the fix, and repeat.
So, all I am saying is that since they have said we will get gingerbread, I'm pretty confident we will, unless there's some kind of unforeseen showstopper bug (which is extremely unlikely). And their dev process is such that we won't get it until people on a whole bunch of other phone models also get it, and at the same time that LG starts shipping new models with gingerbread. Since it is a major change and they don't want to screw over their entire worldwide customer base, they will be very rigorous with testing and conservative on the ship date.
Unless I am wrong and there's the guys from workaholics in a cubicle somewhere in Yeouido poking at Optimus S with a stick.