If your line numbers are the first thing on the line, and they aren’t referenced anywhere in the code (no GOTO type statements) then you could whack all the line numbers with a regular expression in the search/replace box with ^\d+ in the “find” box and nothing in the replace box. When you’re ready to number the lines again, you can go to the menu and do
Edit->Column Editor->Number to Insert
input your starting number, increment, and repeat parameters and NPP will insert your line numbers for you.
If you’re going to be doing this kind of thing a lot, Mikhail’s method and workflow is probably better, but this will do in a pinch.