Hi Edward,
Existing SharePoint Library forms have File Names, and these these are generated somehow by your form, and the forms should store that information. Not unless this was not configured, which I doubt. Now the approach you should take is to disable that auto generate File Name rule/function if the forms already have File Names. Next step, set your submit to library data connection to allow overwrite in order to update existing forms.

Please let me know if this solution works for you. If not, kindly attach a sample of your form so I could have a better idea of what's not working for you.