tried (File --> Form Options --> Preview) did not worked for me. Got error file can not be opened.
After updating main data source (which is web service) of info path form I was not able to see "upgrade existing form" option in version tab .
Able to enable update existing form option by saving template by diff name .
However still I am facing same issue.
Is it because schema has been changed ? I am not deleting any node but adding additional nodes by web service .