No, they literally cannot. The current OpenZFS codebase is the combination of the original Sun/Oracle-owned code and a massive number of new committers. Now, Oracle could relicense the original code, but they can't do anything about the later additions, because they don't own the copyright on those. The only maybe possible carve out is if the code is licensed with something like "CDDL 1.0 or later" then they could make a new version, but I'm not clear on whether that's an option or whether the code is licensed CDDL 1.0-only. IANAL, YMMV. (EDIT: Apparently the CDDL already has a version 1.0 and 1.1? So take exact versions with a grain of salt above.)