Problem: After updating Infopath Browser Enabled Form, the newly added field became read-only on old forms. If you open the existing forms in notepad, you will not see your new field in the xml.
Soln: This happens when you choose 'Do nothing' on version upgrade. To make the fields selectable/editable, republish the form with the following options. Go to Tools -> Form Options -> Versioning -> On version upgrade, choose "Automatically upgrade existing forms". Once the new version is uploaded in Central Admin, go back to your old forms, you should see your new fields as editable.