Enhancement Type

Improve an existing feature

Describe the enhancement

While upgrading a custom server using the GENERIC_PACK environment variable, I ran into the issue that the modpack (GTNH) includes several special files and folders that need to be cleanly replaced for a proper version upgrade (Official GTNH Wiki article).
As this was not achievable using only the tools provided by this project (at least to my very limited knowledge), I suggest adding the possibility to clearly define which files/folders should always be replaced and which should always be kept. Both options are important for modpacks with more complex upgrade procedures like GTNH.