We upgraded both pairs from 7.2(1) to 8.4(1) pretty much without incident. In one pair two rules failed to migrate which appeared to be related to NAT. Otherwise it was pretty smooth. We had one VPN tunnel setup on one pair, not in active use, and the migration seemed to think we had "NAT exempt" in the config, that failed to migrate associated rules. NAT exempt seems to be a misnomer for 'nat 0' or 'nat 0 access-list' neither of which were in the config. If you've got VPN's configured you probably will have more fun migrating the config manually or trying to re-write it in 8.4 beforehand.
Hardware upgrades went well, the failover nodes saw each other. It was interesting when the 8.4(1) active node coped it's config to the standby 7.4(1) node which didn't recognize alot of it. The standby node remailed inactive. After rebooting the standby node to 8.4(1) it was normal.
