We mostly talked about Zabbix server upgrading and rightly so, as the server is the most important part. But what happens when you upgrade the server, do you have to upgrade all the agents at the same time? Maybe you can upgrade the agents first?
Again, this depends on upgrade level. With change level upgrades you can upgrade either server or agent, and it will work. So you can upgrade some agents now, upgrade server later, and some agents a month later.
With minor Zabbix version upgrades, there's a dual compatibility rule:
Old versions of agents are supported
New versions of agents are not supported
Plain and simple, isn't it? The first rule is very helpful when upgrading - we can leave agents in place, only upgrading the server. Agents can be upgraded later when we have time, access, or simply feel like it. It is perfectly fine to use 1.6, 1.4, and even 1.1 version agents with a 1.8 server, as long as you are satisfied with their functionality and capabilities.
But newer versions...