It's a rewrite of the internals, particularly the diffing/reconciliation algorithm. React Fiber involves effectively re-implementing the JS call stack using a data structure called "fibers" to track work that needs to be done.

The question still stands; will we be bumping a version number, or changing a package name?

It's a major version release. v15.5 added deprecation warnings, v15.6 will add a few more warnings and fixes, and v16.0 will be a major release that includes the Fiber internals turned on. (This release path is described in the issues I linked in my earlier comment, particularly https://github.com/facebook/react/issues/8854 ).

It seems like what HopJS does to handle tail calls[1]. It is essentially a trampoline if I understood the docs correctly.

[1] https://pdfs.semanticscholar.org/d10d/aba5df8b4c563b62fd01a3...