InfoPath Schema changes

I've found that if I make a change to an InfoPath form and then update the document within K2 Studio that any existing workflows can't use the new version of the form.

Upon opening the document from SharePoint, the following error message is displayed:
InfoPath cannot open the following file: <filename>
The form contains schema validation errors.

Content for element '{http://schemas.microsoft.com/office/infopath/2003/myXSD/2005-09-08T03:18:17}NewStart' is incomplete according to the DTD/Schema.
Expecting: {http://schemas.microsoft.com/office/infopath/2003/myXSD/2005-09-08T03:18:17}_K2.

This occurs even if the InfoPath document's data source doesn't change prior to integration with K2.net.

Any suggestions as to how we can update our form and yet have the currently existing process instances able to be completed by opening the revised form?

Any help greatly appreciated.
Thanks
David.