Will it be possible to change the ground handler between different airports?
Yes, there will be multiple liveries for ground vehicles, made using the standard FSX format for airplanes. However, we added additional commands to the sim.cfg, which will allow for geographical localization of vehicles, based on the ICAO codes.
Here's a very simple example of what can be done, taken from the sim.cfg of the Catering vehicle:
[fltsim.0]
title=FSDT_catering_01_GGourm
model=01
texture=GGourmet
[fltsim.1]
title=FSDT_catering_01_LSG
model=01
texture=LSG
[fltsim.2]
title=FSDT_catering_01_Cathai
model=01
texture=Cathai
couatl.icaoprefixes = VH
[fltsim.3]
title=FSDT_catering_01_Emirates
model=01
texture=Emirates
couatl.icaoprefixes = OB OE OM OO
Here we have 4 standard [fltsim] sections, just like any other airplane, in this case we have a single base model with 4 different liveries. But, there are Couatl commands added, in this case the "Couatl.icaoprefixes" commands instructs the program about location preferences for that specific livery.
If there's no preference, that vehicle can appear anywhere in the world and if there are many valid alternatives, will be chosen at random. Note that, valid alternatives means correct for the user airplane selected, based on the door's height.
If there's a preference, the vehicle will appear at a certain airport or region, with multiple choices possible.
In this example, we have the "Gate Gourmet" and the "LSG" variations without any preference. This specific catering vehicle has an additional constraint (there's another section in the sim.cfg for them) that will only work with airplanes with a door higher than 2.5 meters from ground. This means, both Gate Gourmet and LSG have a random chance to appear anywhere in the world, but only if the airplane used has a door at least 2.5 meters (smaller airplanes will use smaller vehicles).
Instead, the "Cathai" variation has a Couatl.icaoprefixes set to VH, meaning it will appear only at airports with ICAO code starting with VH (Hong Kong)
The "Emirates" version has multiple preferences set at OB OE OM and OO, and will appear in airports with ICAO codes starting with those letters (Dubai, Saudi Arabia, Oman, etc..)
It's possible to go finer than that, and specify a full ICAO code, like LSZH or KJFK. This will allow to represent handling companies specific to an airport.