If we are saying that the ACE <=> IDN(UTF8) so why is the two headers not in
because there are things which will know about the old headers and change
those, without changing the new utf8 headers.
also if new MUA always use the new headers and only old existing MUA wont be
able to read the new headers will use the old ACE headers, so why MUA wont
know which one is more recent?
because there are things which change the headers, always using the utf-8
headers is not the right thing to do - they might be obsolete.
about the best you can do is require each things that ues the new headers
to check for consistency with the old headers - if they're not consistent,
use the old headers.
in general, it's nearly always a bad idea to have two different sources
of the same kind of information and claim that they're both authoritative.
if you're going to have multiple sources of the same information,
you need some way to decide between them for the cases when they don't