How to use paper prototyping on an existing product

Paper prototyping is a very powerful tool early on in a new product build as it helps you to iron out ux and rapidly iterate.

However if you're working on an existing product and want to test small feature changes, adding fields to a form for example, or changing the layout of features should you consider paper prototyping as an option and if so how? Are there better methods of testing/building iteration...

FYI: This is inside a startup using lean-startup and user testing experiments each week