If you are the sole copyright holder, you are free to issue licenses as you see fit.
The only exception to this is if you issue an exclusive license that effectively transfers (parts of) your copyright to another person. You cannot issue a license for a right that you no longer hold. However, common open source licenses are non-exclusive. The CC0 fallback license is non-exclusive.
Many open source licenses are irrevocable. That means, you cannot terminate an existing license. So if you switch your project to a new license, the old license is still valid for the previous versions, and other people can continue to use old versions under the previous license. The CC0 fallback license (in section 3) is an irrevocable license.
The CC0 has a slight complication. It is not quite a normal license, but
- a waiver/abandonment/surrender of your copyright,
- plus a fallback license, in case “applicable law” doesn't allow you to abandon your copyright.
So in case you have successfully abandoned your copyright, you can no longer use that copyright as a basis to issue a new license. You are not the copyright holder, as there is no copyright.
I am not quite familiar with how licensing can apply to public domain (PD) works (e.g. works for which you have abandoned your copyright). It may be that you cannot slap a license onto PD works.
However, you can certainly decide to stop releasing your new contributions under CC0, and could license them under any other license instead. So some parts of the project may still be in the PD, whereas other parts are subject to some license. It may be best to track licensing on a file by file basis. Even though some parts of the project may be in the PD, the project in whole can only be used under the terms of the license you choose.
