Some units have redundant purposes, one of which is the SOC hazmat truck, it already has a vehicle that does its purpose, it just does it more efficiently than the one that was already present. Most vehicles have some type of special purpose added to them to make them uniquely useful and/or necessary of the ones that were added into the mod.
Appearance wise, the existing vehicles won't be upgraded/changed, but as functions go some of them will gain new features to them which serve some purpose in regards to the mod, most will remain roughly the same though. The unmarked units tbh they really are just there as toys, so there isnt much sense in changing them around any imo, they are redundant to the marked units, the only ones that may grow in numbers are the ESU ones, the rest there are plenty of em.
Terrorism activity will have some aspects added to it, the LRAD (humvee) is a crowd control device, as is the barricade truck. They are to deal with organized protests and things like that. The Bomb disposal vehicle is an additional tool added to correspond to the bomb style incidents and add an additional aspect to the game as an alternative of simply being able to defuse the bomb like clockwork as it was in the original game.
The Tanker is refueling truck, it does have a purpose in the mod, but I won't go too far into what exactly it does as yet. =) But the thing does have something important it can do to make your life easier.
I know you won't tell us what it is but you guys have found/made a script where our vehicles have fuel in them and after so long we are gonna have to "fill er up" aren't we.
*sighs* guess I am gonna have to wait to see when this mod comes out if I am right or not you sneaky cheeky guys!!! ;-)