Requirements for modification to exisiting systems

There has been a lot of literature and discussion on requirements, and how business requirements need to be independent of technology concerns, etc. Also there exist many checklists and templates to ensure that right questions get asked by the development team. The tricky part for me has always been the difficulty in keeping the system context from the business context in case of modifications or upgrades to exisitng systems. I am referring to major enhancements which itself take a small or medium project to complete.
Any thoughts on how to ensure that the thought process is not usurped, or at least tainted by purely technical considerations when trying to understand the customer? Thanks!