The transition issues for IDNA that actually pertain to Internet
protocol behavior have been described repeatedly and are identical
with the kind of changes that were required to adopt MIME.
I can see why you'd want to say that, but it's a poor analogy.
MIME imposed structure on unstructured data by using a subset of values,
and specifically limiting itself to particular technologies. I mean, even
RFC2047 only allowed some particular kinds of data (mostly unstructured)
to be extended, and marked structured data as off-limits.
more precisely, RFC 2047 was intended only for human-readable text,
because at the time we hadn't figured out how to solve the various
problems associated with having multiple representations of
today we have a much better idea for how to do that.
I agree with Dave that IDNA is very similar to things we did with MIME
in both the header and the message body, and I think the experience
with MIME will be a good predictor of experience with IDNA.