change the reply due to a security stand point
Lame excuse that doesn't hold up. If you don't want people to know you're using mirc, get another IRC client. If you don't want people to know you're using 6.10 cause of the exploit, get 6.12.
I just wish it were easier to experiment with some facets of mirc.
It is very easy. The only reason the version reply won't lend itself for that is that Khaled deliberately disabled possibiliities to remove it. That tells me it is his explicit wish that the version reply remains intact (as also stated in the helpfile), so just leave it alone