The defaults directory, imo, is meant to be left alone, so that mIRC's default settings can be retrieved quickly and easily should your working ini files become corrupted.
If you installed to c:\mirc, then your working ini files should be in that directory, although, the usage of //echo -a $mircdir will confirm or deny this.
For future reference, if you use the Update/Upgrade option in the mIRC installer, then your settings will transfer to the new version automatically.