The issue is that you will not be able to sync firewalls with greatly differing versions hence you will not be able to maintain the connection state data after you upgrade one of the firewall pairs.
You can sync an 8.0(2) with an 8.0(4) firewall in my experience though so you would be able to perform a seamless upgrade in that situation. I have personally not tried to intentionally sync an 7.2(4) firewall with an 8.0(4) firewall. I wouldn't hold much hope for that working if Cisco already told you it is not supported.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...