I'd be interested in the answer to this too. I also read that you have to pay for the upgrade of each Workspace. On the Trello website it says:
Standard and Premium upgrades happen per-Workspace, rather than per-individual-account, which is why we suggest trying to organize your boards and members into a single Workspace. Premium provides lots of additional features, such as integrations with other services, collections, and a host of features to make it easier to manage your Workspace.
See Also
10 Ways To Energize Remote Teams