You can move a role from one circle to another through a Governance Meeting of the supercircle that both circles belong to. (Premium users can also use Asynchronous Governance Proposals to do the same thing.)
In the following example, Tree is the supercircle and Poplar and Yew are the subcircles. We are moving a role from Poplar to Yew.
In the Governance Meeting of the supercircle, you will first need an agenda item to move the role out of the first circle.
Restructure the Sub-Circle that has the role, as shown below:
After you accept that proposal, you will need an agenda item to move the role into the second circle, as shown below:
After you accept that proposal, the role will have moved from Poplar to Yew.