Question about installation of the update:
In my mods folder I have something named "bko_vw_t3_base_1"
That sounds like the first version of the mod from 2021. Version 4 isn't 100% compatible, but I expect that for driving vehicles you'll see no issues. For the assets I'm a bit unsure if you'll maybe miss a few parts and find some place holder boxes around them, or that it is only that you can't access the model settings to adjust them anymore.
It's probably best if you keep the old version of the mod active for your existing map and add the new version after it in the load sequence. You may find that you have a double menu entry in the asset menu then. For that it's easiest to just change the availability years in the construction file of the old mod to be ending before 1850.
You'll see a big improvement on the v4 models versus the v1 ones.
Please let me know if it works or if you need help.