One of the cool things about doing replace-in-place is that you can A&#x2F;B test the old and new versions. We are doing that on our current Form Builder rewrite at JotForm. We rewrite one piece and make it live to 50% of users. Then we receive daily morning emails about each test. If some metric is in red, we discuss (or watch fullstory, or talk to users) what might be causing it and improve the new version.<p>Here is a fresh example, we released the new version of PayPal payments pro&#x2F;express integration, and the success rates stayed in red. The old version was beating the new version. It was 3x better even though almost everything was same. After some head scratching, we found that the old version had a link to a direct PayPal page where the can get their api credentials, and the new version was missing it. From there, the fix was easy and things turned green.<p>This is a story that has happened over and over again. When you rewrite software, you lose all those hundreds of tiny things which were added for really good reasons. Don&#x27;t do it blindly.