It's much more easy and less time consuming.
1) Navigate to 'Content', then 'Cars'.
2) Click on any car on the left.
3) Click the tiny arrow next to 'Update Previews' at the bottom of the window, then click 'Options'.
4) Select the new preview you'd like to use at the bottom of the preference window, then click the 'Update Selected Skin (without confirmation)' button (box with a + in it).
This tells Content Manager that this is the default skin to use.
5) Quit the preview, and select all cars in the left list (click one, then CTRL+A).
6) Click the drop-down at the bottom and select Look > Update Previews.
7) Click 'Update XXX cars' when you're ready. This may take a few hours depending on the number of cars.
Or
1) Go to 'Content', then 'tools'.
2) Click 'Launch' button of the 'Update previews' tool
3) Click 'Autoselect' button at the bottom
4) Click 'Update Previews' at the bottom of the window
View attachment 1462359