Hi Jovix,
Welcome to the board!
If you upgrade the schema for the forms then opening one of the old ones will require that the new schema nodes are added. You can do your own custom OnVersionUpgrade, but anytime you change the xml, it could require re-signing the document, since the signatures are protected with a checksum on the form and if the form is changed, the checksum no longer works.
One option would be to create another form library with the new version.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra