Procite databases can be opened with some versions of Endnote, but not EndNote 8. See http://www.endnote.com/support/faqs/Import/faq3.asp for further details of compatible versions.
From experience I can say that EndNote 7 opens Procite libraries well. I found it helpful to do a test conversion to look at how the data went across and made use of the ability to custom map fields from Procite to fields in EndNote for a smooth transition.