SharePoint supports promoted properties. Open your form in InfoPAth Designer, go to Tools->Form Options and choose Promoted Properties. Select the values you want to appear in your list. Install Qdabra's qRules with support to map XML to SharePoint lists. Define a mapping that updates one of the existing 250 rows based on some unique ID. Open and submit one of the closed forms to test. After you have them all working, you can continue manually (which probably would take an hour), or you can use the batch tool that comes with qRules to upload lots of XML into lists.
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