In Acrobat :
1) Open the form
2) File > Save a copy...
3) Open the copy just saved
4) Make your changes to the PDF
5) Save as Other > Reader Extended PDF > Enable More Tools (includes form fill-in and save)...
FormsCentral saves a Reader Extended PDF by default so that the PDF can be filled out and saved in Adobe Reader. This has the side effect to lock the PDF from editing. The "Save a Copy" removes the Reader Extension and allows you to modify the PDF.
Note that some changes might break the FormsCentral workflow.