When peering VPCs with their Atlas VPC, users are able to modify their group's Atlas CIDR block if there are not any active clusters within their Atlas group.

However, it's a strict requirement that the Atlas CIDR block end with /21. If users attempt to modify the Atlas CIDR block to end in anything other than /21, they will receive the following error:

Atlas CIDR must be a /21 block

It may be important to include this requirement within the Set up VPC Peering Connection and VPC API sections of the Atlas documentation.