Yeah theres no way around the download, but if you swapped the IDX and downloaded 20 it wont work as the launcher thinks the game is complete but it isnt, then you will have to swap back the IDX files and the launcher will have to download 150 instead 170… 170 - 20 = 150
Basically you have to download it all as far as I can tell.
Swapping the files back just unbreaks the file manifest.