If they don't have silEnT GUIDs that is what happens. If they have it, i.e. the silent.dat file, then it doesn't happen. If the instalation is completely separate, you can copy the old silent directory to the new installation loation. The silent.dat file alone is enough to keep the old silent GUID. After that, you can use the !updatepbguid command to update their PunkBuster GUID to the database. I think ET: Legacy is reusing the cvar reserved for the PunkBuster guid, i.e. the cl_guid field.