I asked support for help with this, and Gino Sandoval Purata replied with this incredibly useful answer, so I thought I'd share. The third method allows you to rename a board even while you've got an ongoing sprint but deactivating and reactivating the Sprint feature in project settings.
When a sprint has not been started what you see is the board and you can rename it as it is shown in the next video.
Renaming Scrum next-gen board.mp4
When a Sprint is in progress what you see is the Sprint name, the way to change its name is as shown in the next video.
The workaround to change the name of the board while a sprint is active is by disabling first the sprint feature as is shown in the next video.
Board rename workaround.mp4
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