these licenses are irrevocable and you can't just "backport" additional restrictions
https://creativecommons.org/faq/#what-happens-if-the-author-...
For CC By-Sa works the new one might also not be strictly less permissive, because applying the restriction should allow people deriving derivatives to do the same via the share alike clause when they previously couldn't (though I didn't actually check the wording this person is using so this is just assuming the restriction is applied like I think it ought to be).
AFAICT the only thing that could possibly change this is if he explicitly passed his "moral rights" to another person, but obviously that doesn't apply here. CC-BY 4.0 contains this clause, but I don't think you can just be made to "agree to waive" these rights, as "to the extent possible" hints at:
Moral rights, such as the right of integrity, are not licensed under this Public License, nor are publicity, privacy, and/or other similar personality rights; however, to the extent possible, the Licensor waives and/or agrees not to assert any such rights held by the Licensor to the limited extent necessary to allow You to exercise the Licensed Rights, but not otherwise.
Another point is that any updates to existing works will only be available under the new licence. This might seem irrelevant to drawings and paintings, but the author has on occasion made changes to his older works for one reason or another, so at least the resulting newer versions would be protected by the additional terms (assuming they’re legally in the clear, of course).