Now that we've developed 2to3 and 3to2 I've started wondering "what next?" and realized that we could use a 3to3 tool to apply the relatively few and relatively simple sets of code changes to deal with library reorganizations, renaming, parameter changes, default behavior changes, other deprecations that have an equivalent, etc.  The list of migrations to go between any given 3.x to another 3.x release would likely be pretty small.

Just tossing the idea out there incase someone is interested.  I expect the hard work is already done.