If you have an administrator approved template for InfoPath in SharePoint, you will notice that whenever you upload a new form template via Central Administrator, all existing instances of the form would not be automatically upgraded.
If there are specific scenarios where you need to have existing forms upgraded, there is a very simple workaround.
1. Take note of the most recent uploaded version of the form in Central Administration - General Application Settings - Manage form templates.
2. Download a copy of the form instance.
3. Open the .xml file in a plain text editor.
4. Modify the version in the form's solutionVersion attribute near the header.
<?mso-infoPathSolution name="urn:schemas-microsoft-com:office:infopath:Your-Form-Name:-myXSD-2014-01-27T23-22-27" solutionVersion="1.0.0.155" productVersion="14.0.0.0" PIVersion="1.0.0.0" href="http://Your-Site/Your-Form-Name.xsn"?>
5. Save and re-upload the .xml with the same name.