I have recently upgraded to Adobe Acrobat Pro XI from an earlier version. When I tried to
modify a fillible form in Adobe FormsCentral which was originally created in the earlier version
of Adobe Acrobat Pro, I got an error message which said, "FormsCentral could not import the PDF
document because it is an XFA form.
What do I need to do to be able to modify this fillable form without having to create it from scratch?
You need to use Adobe LiveCycle Designer to create and update XFA forms. You can't import this type of PDF in FormsCentral.
Live Cycle Designer used to come with Acrobat but now it is sold separately.
You might want to ask your question on the Acrobat Form's forum to know how you can modify your XFA forms after updgrading Acrobat.