All of the stacking benefits will be added to the Enterprise plan automatically.
If you wish, you can split your codes into separate accounts (called Agency Workspaces). Then upgrade one of those accounts, so you would be able to share the Enterprise campaign entry limit and social profile limit with all of your Workspaces in addition to the entries and profiles you originally had. It's quite flexible how you use your new limits.
However, only the one Workspace that upgrades to Enterprise would get the additional features that are exclusive to Enterprise plans, such as the custom campaign domain.