We had a previous version that we reinstated (was stored on the network), so the correct version is back in the library with the old name for the field (at the moment struggling with trying to get the right access to the site to see what the vesioning settings are)
I understand that the data will no longer display if you eg change animalName to petName as the template won't have a clue that you wanting it treating the way previously.
However as the data for the forms are stored separately in the xml file I would expect that if I had not opened them and resaved them, then the data would still be viewable if opened in notepad. Thes files have not been edited and are missing this information which is the bit I don't understand. I have checked that they were created by the same version of the template and that is true and also the url of the restore templated is the same as that referenced by the form.
The data I changed was I contact picker which fished out 3 fields from Active drectory and populated the form. I changed the name of one of the fields that got populated. (not one in the person group for the conact picker)
I would have expected that the other fields that had not been changed and their structure had not been changed to still retain their data at least.
The other areas of the form have retained their data, just not area related to the first contact picker.
Examining the xml files in notepad . any new ones that I create have a nice structured layout, however these previous ones have no formatting at all, just one long wrapped line.