https://docs.atlas.mongodb.com/scale-cluster/#migration-downtime-and-performance-impact

This seems a bit too soft
"Some configuration changes, such as changing the instance size, require migration to new servers. Depending on the amount of data to migrate, migrations can take a significant amount of time."

Suggest something like:
"Making changes to an cluster (like the disk size, IOPS, or instance size) will likely involve migrating to new servers and/or volumes, which can take a long time. However, in some special cases we're able to make certain changes more quickly."

Not that exact wording, but something along those lines