You shouldn't have needed to reset SIC. It definitely survives upgrades. During the upgrade on a firewall, services stop. Eventually the firewall reboots. You won't have any communications with it until it's done rebooting to the new version.
I highly, highly recommend upgrading with CDT, either on the command line or in SmartConsole. Once you have your management upgraded to R81.10, connect to SmartConsole. Right-click on your cluster object, go to Actions, then pick Version Upgrade. It handles updating CPUSE, distributing the CPUSE package, upgrading one member, enabling cross-version sync, failing over, then upgrading the other member. Seriously, this is my favorite new feature from Check Point since the initial release of R80.
I've upgraded around 70 clusters this way. There are occasional snags during the upgrade process, but those would have been hit during a manual upgrade, too.