The question is rather loose in defining context as well as in used terminlogy.
But generally, if the the fields are added to an Infopath form template through Infopath Designer 2010, then the new form tempalte was created and only template with which previous forms were created can be used for editing/updating previously filled Infopath forms.
The correct solution is to put Infopath form templates into Infopath form library separately from a document library into which Infopath forms with data are stored and/or save changed form templates under different names. Otherwise, if the new changed (aka modified aka updated) Infopath form template is published into the same library where the documents or Infopath forms with data are stored, then changes to Infopath form template will overwrite the old form template and editing/changes of previous forms with data will not be possible while the old forms will be shown according to new changed form template.