Disclaimer: I'm not a lawyer. But I have some experience with licenses from a software perspective. Still, this is not legal advise.
Additionally you can't just 'update a license' for any items already released, the licenses are stuck to them
The original author(s) can of course always change the licensing terms as they see fit, especially when they introduce changes. They wholy own the work, after all. However, earlier versions of the work could still be distributed under the old licensing terms (but modarchive doesn't have to carry both versions).
While maybe not that common in the music field, this does happen in the software world, with software projects changing license after all copyright owners (i.e. people who contributed non-trivial, active code) agreed on the change. Depending on the number of copyright owners, this can be relatively difficult, though.
however we will consider inserting the CC0 license as an option
Me, I'd really appreciated that. especially since the "Public Domain" license was retired for a reason. It's not really applicable to many jurisdictions, since they might not have the concept of a public domain. And even in the US,
putting something into the public domain like that isn't really possible. Things
fall into the public domain regulated by some strict rules instead. In short, the license might not hold in court, with the original author possibly being able to argue that the terms are void and a third party shouldn't have been able to use the work as they did. Yes, highly unlikely to happen, but still a theoretical possibility.
That's also why CC0, the recommended "public domain" license, has lawyer-approved language that gives the license a fallback with terms that effectively make the work "public domain", even if that concept does not exist in the target jurisdiction or the work wouldn't normally fall under it.
Moreover, the retired "public domain" license is laking explicit waiving of implied warranties, which are a thing in certain jurisdictions (the US included). Unless waived, you guarantee the work is fit for its advertised purpose, free of severe faults, etc. That's what the common "AS IS" blurb you so often see is for.
So, to recap, I'd really like to see a CC0 option for uploads (and the old "public domain" option to disappear). A way to change a license would be great as well, maybe with a visible history (i.e. license X from date - date, license Y now), but I can see how that might be difficult. But letting an author update the old "public domain" license to CC0 should be workable, since CC0 is what the "public domain" license was supposed to express anyway.