Hopefully it should be a pretty easy transition. The plugin is enabled by default, but if you are using vehicle classes in C++ you will need to add a dependency to the PhysXVehicle module to your Build.cs file, so the build system knows about it. The only thing that might need manual work is converting from TireType to TireConfig assets. Here are my draft notes on the topic:
Note: a manual step is required if you want to migrate old ‘TireType’ assets (deprecated in Engine) to the new ‘TireConfig’ assets (defined in the new PhysXVehicles plugin). There is a new console command available in the UE4 Editor that will do this for you - simply summon the console and enter ‘ConvertTireTypes’. This will create new TireConfig assets, copy over properties, and assign them to any VehicleWheel Blueprints. The old TireType assets can then be removed.
In the old system the PhysicalMaterial had a list of TireTypes and their friction scale. Now the TireConfig has a list of PhysicalMaterials with friction scale for each. This is because Engine (which contains the PhysicalMaterial class) cannot depend on classes in the PhysXVehicles plugin.