Editing Forms in Acrobat XI?

My problem:

I have a valid license and serial number for Acrobat XI Pro.  I must maintain form documents created in versions previous to Acrobat XI Pro. It is my understanding that XI Pro does not support the Forms Designer that was used to create the documents. I therefore assumed I must use a version that does have Forms Designer. I attempted to downgrade to Acrobat Pro X (that does include Forms Designer) and was able to modify my older documents but the trial has since expired and the serial number for XI Pro will not activate X Pro. I need a serial number that will activate the older version of Acrobat.  Adobe customer support  has informed me that I cannot downgrade to Acrobat X Pro.  What are my options?  Do I have to bite the bullet and buy LiveCycle Designer ES4?  Will that allow me to modify the older documents?  This is ridiculous.