You need to use the free MSI editors like ORCA or SuperORCA. Microsoft use to provide ORCA tool in their service pack. Not sure if they are continuing so. SuperORCA tool you can find at http://www.pantaray.com/msi_super_orca.html. Now when I had similar task to achieve following was my approach. I used SuperOrca to open my MSI file.
1] On repair/remove MSI dialog screen, add one more radiobutton for "Modify" - In SuperOrca-->Under RadioButton table --> Add entry for Modify radiobutton
2] Rename BackButton's text to "Reinstall" from its original "Back" --> In SuperOrca-->under Control table --> change Text property to rename Back to Reinstall
3] On selection of Repair/Remove radio button enable Finish button and disable Reinstall button. On select of modify radio button enable Reinstall button and disable Finish button - In SuperOrca--> Under ControlCondition table --> add entries for enable/disable "Reinstall" and "Finish" buttons for each comibinations for repair/remove screen (In my case dialog form name is "MaintananceForm")
4] Now from SuperOrca tools ControlEvent table find out conrol name for BackButton, In my case it is "PreviousButton". Now modify its Event, Argument & Condition entries from the table. And the values for these properties must be the same for the "Next" button properties from the Welcome screen of MSI file which redirects user to show installation options to choose.
Note: This is working fine for me. But when you re-select installation options again and click Next, somehow it does not forced for installing the selected option. Instead it repairs the MSI. I tried deleting entries from Windows\Installer folder so that MSI will not pick cashed installed version but no luck. If anybody is having answer on how to forced MSI to re-install through MSI setup wizard please let me know.