Developers change the code of their software to add new features, fix bugs, or enhance its structure. Such frequent changes impact occasionally the security of the software. This paper reports a qualitative study of the practices of changing secure-software in the industry. The study involves interviews with eleven developers and security experts working on banking software, software for control systems, and software consultation companies. Through these interviews, we identified that the main security aspects are: dependency vulnerabilities, authentication and authorization, and OWASP 10 vulnerabilities. The common techniques used to assess software after code change are: code review, code analysis, testing, and keywords search. The main challenges that practitioners face are the diversity of the security issues and the lack of effectiveness of the security assurance tools in detecting vulnerabilities. The study suggests that developers of secure software need techniques that support effective security assurance of modified software.