Repackaging: Best practices while converting 32-bit application to 64-bit application
Community Chosen Answer
Your question is a little unclear because you preface it with 'repackaging' but you ask about converting an application.
If you're asking about modifying an MSI, there is nothing to do. 64-bit apps anticipate 32-bit apps to be installed and act accordingly. If you're asking about converting the program, that implies changing the application code to work as a 64-bit app. For that, you'll need to read more about programming.