It really isn't that simple. Both versions can exist for one. For another, if it did that, you could never choose to join the UTF8 version of the channel if the non-UTF8 version existed. That's just two examples why you can't do it the way you suggest. It would also be a pain to manage all of the events that need to track whether or not to use a UTF8 or non-UTF8 channel name.
There may be options available for IRCops that will let them manage the "outdated" channel names at some point. In the meantime, mIRC 6.35 works just fine. Almost the entire reason to upgrade to mIRC 7.1 is FOR Unicode support. If you don't want or need it, there's not much reason to upgrade from 6.35 at this time. IRCops can easily stick to that without any issues until either mIRC offers an option for them to handle those non-UTF8 channels/nicks or they update their servers to no longer use non-UTF8.