>>>>> On Thu, 17 Dec 2020, Mike Gilbert wrote:

> Doesn't the same restriction apply to relicensing it?

No, because the CC licenses have an explicit provision that allows it
when distributing a modified work (which they call an "Adaptation",
defined in section 1a).

For example, CC-BY-SA-3.0 says in section 4b:

   You may Distribute or Publicly Perform an Adaptation only under the
   terms of: (i) this License; (ii) a later version of this License with
   the same License Elements as this License; (iii) a Creative Commons
   jurisdiction license (either this or a later license version) that
   contains the same License Elements as this License (e.g.,
   Attribution-ShareAlike 3.0 US)); (iv) a Creative Commons Compatible
   License. [...]

Item (ii) is what gives us the right to distribute under CC-BY-SA-4.0.

Ulrich