Yes, 100% there will be. Ok, so let me explain in layman's terms the differences, just in case if you were not clear.
Management server - separate entity used to manage CP gateways
Single gateway - just one firewall, no cluster
Cluster - 2 or more firewalls either in HA or load sharing
Standalone - Management + gateway in one entity
Now, as it sounds like you have standalone, any time there is upgrade, reboot is needed, meaning there will be downtime, as any traffic going through it wont work for the time during reboot. One thing to remember, its possible device may not fetch latest policy, meaning it may default back to initial one, which blocks most things, so make sure you have physical access to unload it if that happens by running fw unloadlocal command
Andy