Although Desktop Pro/Premier can be converted to Enterprise, there is no Intuit provided method of going back from Enterprise to Pro/Premier. If all you needed were lists you could easily export/import those. Transactions, you might be somewhat successful exporting to excel and then importing those but there are services that can do the conversion for you.
I have no affiliation, but Big Red Consulting offers the exact service you are looking for $249. (add ($49 for rush). He is also a respected long time contributor here and as such his services are what they are presented to be.
