You can take note of current values and restore them during configuration time.
Thats not the point
Sure can backup, note the valuies but that does not matter
The default answer "YES" for the question "overwrite?" makes no sense
Its the same let it run with the -d option and restore a backup, press enter at all other questions and type NO here - The point is that the script takes notice that there is a changed "nat.conf" and here is "overwrite" simply an "option from hell"