We can perform in-place and out of place update to upgrade the Pega 7.2.2 to Pega 7.4.
In-place requires complete downtime and application cannot be used when the update is performed.
Alternatively, we can perform out-of place update which minimizes the downtime that is required. we cannot completely eliminate the downtime but it reduces the downtime to greater extent.
In an out-of-place update, an offline migration rules schema is updated, while the data schema is updated directly. The offline schema can reside either on the production database or on a second temporary database. These updates significantly minimize down time because they modify an offline schema and do not update the schemas in production.
As a best practice, use out-of-place updates for split-schema configurations.
You can refer the below update guide for Pega 7.4 which will provide detailed information on performing the update.
In Out of place upgrade, we need downtime for 'data only upgrade'. The 'rules upgrade' doesn't require any downtime as it could be done out of place with either single or two database. So minimizing the downtime.
If your machines are hosted in Cloud like AWS, Azure, Google you can think about Blue/Green deployment. It is easy to create new instances where you can perform upgrade and test. Finally, you will need to sync data from the working db to the new one. In the end, you need just change on the Load Balancer traffic from old instances to the new one.