The pre-requisite here is that you first enumerate a fully complete spec of all behavior that your clients believe it should have. This seems too high a burden for what you're trying to achieve.
My experience has seen it work well as either 1) bottom-up: pick a small enough section of the codebase such that you can fully understand it, then rewrite just that piece, repeat until complete to get an exact copy of your existing application or 2) top-down: you write a new application from scratch completely, starting with understanding what your business's goals are and how to best serve your clients
My experience has seen it work well as either 1) bottom-up: pick a small enough section of the codebase such that you can fully understand it, then rewrite just that piece, repeat until complete to get an exact copy of your existing application or 2) top-down: you write a new application from scratch completely, starting with understanding what your business's goals are and how to best serve your clients