I keep getting a message to change to a new URL
https://boinc.bakerlab.org/rosetta/
but no further information on how to do this.

The project has enabled https, secure network connections, so the only change in the URL is the "S".

Lots of info. here.

Important points:
attach with same profile and password as your original account. i.e. select the option to connect with an existing account, not a new one.
You must first detach the project under the existing URL without the "S", and this will abort all of the tasks you currently have, hence the discussion of setting "no new tasks" (NNT) until existing work completes and is reported back.
I would keep it simple and ignore the manual editing of files that some describe in the thread.

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Rosetta Moderator: Mod.Sense