You can edit the PDF without breaking the submit process or losing existing responses as long as you do not modify the form fields, if you are just adding copy to the pages but not to the form fields it should be fine.
Here is an FAQ on editing FormsCentral PDFs in Acrobat (I understand this was an imported PDF): http://forums.adobe.com/docs/DOC-3661
Another option would be to export the responses from that form, edit the original PDF the way you want it and then import this new edited PDF as a new form in FormsCentral where you could import the responses from the first form.
Let me know if you have any follow up questions.
I have tried both options. If I follow the instructions in FAQ, I have no problem editing the form, then saving it. But the changes do not appear in the form that is on Forms Central. Am I missing something?
If I follow the instructions to export the responses and create a new form with edited copy, I don't see how to import the responses from the first form.
The changes would not be reflected in your form online but that shouldn't really make much difference since you can't make changes to the fields themselves and that is all that is shown in the form online. You wouldn't re-download a new PDF (since it wouldn't have those changes), the PDF you've downloaded and are editing would be the one you'd want to distribute.
If you are changing the fields in any way you would have to upload this new PDF.
I am sorry, I mis-spoke about being able to re-import those responses. You could however copy and paste them (depending on how many you have) into the new Imported PDF form in FormsCentral. Open the original form in FormsCentral and select and copy the rows in the Response Table and open the new form, insert enough blank rows in the response table and paste in the responses.
OK. Thanks very much. That clears things up.