You will need to edit the forms in LiveCycle Designer. Even if it had not been created in LiveCycle PDFs are not editable in FormsCentral.
The FormsCentral Import PDF feature does not enable you to edit a PDF form, your can import many PDF forms and then download a new copy of the PDF that has a "Submit" button that submits the responses back to FormsCentral so you can view and analyze them there, but does not enable a web version of that form or provide any capability to edit the form.
LiveCycle forms are different than Acrobat PDF forms and do require LiveCycle designer, this write up on the difference from the LiveCycle team may be helpful.
Thank you, Josh, but, well, therein lies the problem.
I upgraded the Technical Communication suite from 3.5 to 5, and LC is no longer there.
What now? I don't want to buy LC when I never had to before . . .
I don't know anything about that suite but I am guessing that 3.5 might have included Acrobat X (or earlier) which included LiveCycle Designer and now 5 includes Acrobat XI which does not include LiveCycle Designer any longer, if that is the case please see this document on how to obtain LiveCycle Designer: