What you need to be looking at here, is Package Configurations. If you right-click the control flow pane, the 2nd or 3rd option is Package Configurations - that launches a wizard that helps you set up configurations for your SSIS Packages.
I've only made use of XML configuration files so far. I have used them to change connection strings, file locations, variable values etc. etc. The configuration values are loaded, as I understand it, at run time of the package, so the new values are applied then. Also, it seems to make life easier when you turn off validation (set DelayValidation to TRUE), to avoid unnecessary validation errors.
HTH