When I was working on an ActiveX control embedding a video stream into a web page (a long time ago when others were still working to prevent millenial apocalypse), we had a customer who used the "coffee cup" test - put a coffee cup down on F5 so it would continually refresh. Extremely harsh!

Michael Feathers wrote a great book "Working with Legacy Code" for exactly this. Step 1: wrap anything you are going to change in tests so you know what it does now and can ensure it continues to do that after your changes... Step 2...whatever is necessary to fix problems you are having. The strangler fig pattern can be helpful - gradually wrap/replace sections until the whole codebase has been replaced, like a strangler fig strangling a tree. Ian