I think that there are two things happening here.
1. we are trying to indicate that the current chair becomes the chair of the Hibernating WG
2. we are trying to indicate that the chair will change from a voting member of the EC to a Non-Voting member of the EC.
separating the idea into separate sentences may be a better way to note that.
Change: "Hibernating WG Chairs become Non-Voting Members of the
Sponsor after their WG enters hibernation. The Sponsor Chair may
appoint new hibernating WG Chairs to replace vacancies as soon as
practical, subject to confirmation by the Sponsor at the next
plenary meeting."
To: "A WG Chair becomes the Hibernating-WG Chair after their WG enters hibernation. The
Hibernating-WG Chair becomes a Non-Voting Member of
the Sponsor.
The Sponsor Chair
may appoint a new Hibernating-WG Chair to replace a vacancy as soon
as practical, subject to confirmation by the Sponsor at the next
plenary meeting."