Hi,

every saved infopath form (xml file) in his header has the link to the XSN
template that is created from, so when you open the XML file, he loads the
values from the XML and render on the screen according to the XSN file
(template). When you change the XSN, than old XML files are rendered with the
new XSN file, so if you change the dataset of the infopath form, (add new
filed or etc.) you will not be able to open old XML files.

The workaround is to publish the for to the FormLibrary, and the copy the
xsn to some subfolder, named like templates or else, rename the template.xsn
to templateV1.xsn and templateV2.xsn and so on depending on the version of
the form. Then you have to open a new form from the last published template
with some web part that use OpenXMLDocument SharePoint function. After the
form is saved the link to that version of form will be saved in the XML, and
old XML files will have the link to the old version of XSN.

Darko