Although some people use older versions due to scripts or simply not wanting to take the time to upgrade, I don't think you'll see anywhere near 90% who are on 6.35 or under within a week of mIRC 7 being released. I'd be willing to bet that at least 75% of mIRC users will upgrade. And with the changes to unicode that can cause the issues you mentioned, that may increase the percentage who upgrade even more.
If you're on channels that use codepages in order to communicate, I'd recommend trying to get everyone to agree to upgrade to mIRC 7 or to use another client that offers unicode. Other than script issues, there's really no reason not to upgrade.
