Systems that have both persistent disk stores and a bidirectional WAN gateway to replicate data among sites can implement an upgrade that does not require system down time. It requires multiple reconfigurations during the process, so that each site can be individually upgraded while the rest of the system continues.
This procedure will not work reliably for you if any of your WAN-connected systems includes clients that use continuous queries or subscriptions. In those cases, upgrade each system separately using the procedure described in Upgrade from Version 8.2 to Version 9.
This procedure assumes that each site has already been upgraded to Pivotal GemFire version 8.2.3 or a more recent 8.2 version.
This diagram shows the initial state of the system.
The procedure for upgrading two sites, Site A and Site B: