I migrated from Winhelp to CHM files some years ago although
I haven't felt the urge to go to Webhelp yet
I'd have to question why you want to migrate to CHM
when you'll be going to Webhelp anyway. You may be asking for a lot
of sleepless nights and additional work. Of course you may have a
very strong business reason for doing this so I'll shut up! When I
migrated I just created a new project and imported my Winhelp file.
It did a pretty good job of doing this.