Not every environment opts for VMware DRS license. While LCM upgrades rely on resource scheduler it is still possible to achieve success in hypervisor upgrades without DRS feature.

Things to consider as part of preparation for the upgrade:

1. Compatibility matrix.
2. Redundancy factor (must be at least RF-2 for a non-disruptive upgrade).
3. Third party applications dependencies. For example, where Zerto Z-VRAs are present, additional tasks may be necessary to avoid disruption to live replication.

Refer to vSphere Administration Guide for Acropolis (using vSphere FLEX Client): ESXi Host Upgrade Process  for detailed process guide.