I got Nick1 set as primary nick in options. I got Nick1_ set as secondary nick (alternative). I go into options and change the alternative nick to Nick1-, press OK and my nick changes to the alternative nick value. Looks like a bug to me, since I only changed the value for alternative nick, I don't want the actual nick to change to that.

I tried with different values and everytime I change the alternative nick and press OK my nick changes. Would be nice to fix this kinda behaviour, because sometimes because of this I loose my primary nickname (I change altnick, my nick changes to it and this second my primary nick gets taken).

mIRC 6.31 here.

Whilst using Primary nick, go into options, change alternate nick, click OK.

Nothing overt happens, namely mIRC doesn't change my current nick.

I managed to reproduce it exactly once. After that time, mirc never did this again, even after reconnecting, restarting mirc and trying other little things (basically I tried repeating some of the things I'd done in my first session, like connecting to additional networks). It's weird.

Before anyone mentions it, that one time it happened, my current nick was not set to my altnick (it does happen in such cases, presumably intentionally).