From what I understand, there are a limited number of slots available for the upgrade. Tesla is putting the first upgrades into the most efficient vehicles first. This means that they will automatically upgrade available cars when they are in the service center for other services. Very manpower and logistically efficient to upgrade those cars that are already going to be in the service bays.
They will probably upgrade the 2.5s first, get a solid read on how it is going and slowly proceed through the entire fleet.
This will allow the technicians to become familiar and efficient about doing these upgrades.
Imagine US will be first, Canada shortly behind and Europe/Asia after that. Believe that each countries requirements for validation will enter into their decisions of where to go first.
Interesting to see that Tesla is independantly starting to do these upgrades on their own, with no prompting required.
For some reason, it seems that Tesla wants as many of these V3 chip equipped cars on the road as possible. Perhaps they allow quicker data gathering for their Autopilot and TeslaVision.
Do not know of any other manufacturer that is pro-actively installing upgraded computer chips into their existing fleet of products. Amazing.