Just as a note on those channels, the ops can easily join the UTF8 version of those channels and slowly work to move people to the UTF8 version of the channel. This would really be a good idea even if mIRC adds some easy way to continue using the non-UTF8 channels. It would be easy to set up the UTF8 channel and then ask people to move over there. The ops can then leave a "bot" in the channel that will tell anyone who joins the non-UTF8 channel to use the UTF8 channel instead and could even /invite the user to the correct channel. Eventually, more clients are likely to start using unicode and you'll start running into more problems with users unable to access the non-UTF8 channels. It would serve in everyone's best interests to start making the move.
Yes, changing the servers themselves takes more work, but changing a channel is fairly easy to do and considering you're keeping the same name (just encoded differently), you aren't sacrificing anything.
It was much more work for me to update the Invision script to support unicode/mIRC 7.1 than it would be for me to move everyone in one of my channels to a unicode version of the channel if I had such a channel. And I would have already made that move if I had that kind of channel. It would have been done before mIRC 7.1 was ever released because it was very well known to anyone following beta that this was coming.