First lets make sure we are on the same terminology:
"Form Fields" are those fields in your form template that, when editing a form, you enter data into. These are saved as part of the form XML or to a database.
"Promoted Properties" are those fields which you specify in your form template should be promoted to SharePoint to be listed alongside your forms.
I believe you are referring to promoted properties here.
Promoted properties are sync'd on SharePoint whenever you "publish" your form to SharePoint. At the point of publishing the new form template, SP looks to see what fields are being promoted, and adjusts its current set of fields (i.e. adds or removes as required). When you update your template and change what properties are promoted, and then publish again, SharePoint is "supposed" to do those adjustments automatically. HOWEVER--it will not go through all your existing forms, saved to SP, and update the properties on those. This can be done by saving off your existing forms, and uploading them back to SP. At that point, SP will reprocess that XML file and pull out the appropriate fields to be promoted.
If this is not happening, then the problem most likely lies with the SharePoint installation. I don't believe there is any configuration where you can turn the feature off... so something else is wrong.