We use the .NET interop DLL in SSIS packages for FTP and SFTP, is installing version 5.5.3 enough or do we need to also register and upgrade to the latest Interop DLL?
What do you mean by ".NET interop DLL"? Do you mean WinSCP .NET assembly? You always need to upgrade that along with WinSCP. You cannot use different versions of WinSCP and WinSCP .NET assembly together.