How to keep the connections when perform firmware update? 2 Fortinet 100e firewall with HA

We have 2 Application servers that have some applications connect locally to pg-pool on the same server. 2 Postgresql Databases servers, stacking switch between AP servers to firewall and stacking switch between firewall to DB servers. When we updated the firewall firmware, we have about 1 minute downtime for fail-over process between the firewall devices. After that, the pg-pool connection to database server disconnected once and re-connected after that. But the application still timeout and disconnect. I would like to know if there is a way to update firewall firmware on 1 firewall without network disconnection? Thank you!