A dedicated bandwidth can only be used by one EIP. An EIP can only be used by one cloud resource, such as an ECS, a NAT gateway, or a load balancer.
A shared bandwidth can be shared by multiple EIPs. Adding an EIP to or removing an EIP from a shared bandwidth does not affect your workloads.
A dedicated bandwidth cannot be changed to a shared bandwidth or the other way around. You can purchase a shared bandwidth for your EIPs.