There is no in call menu option to become chair, you have to enter the conference as a chair to be one. Have you considered using the same numeric ID for chair and guest, and then use a different PIN for each? Because you put a PIN on the conference, everyone will be asked to enter a PIN to access the conference, the MCU doesn't know who is supposed to be chair or not. One other option is to use different conference IDs for chair and guest, chair would have a PIN, guests would not.
If you'd like to have the ability to use the in call menu to change from a guest to chair, I'd suggest you contact your Cisco account manager and file a feature request. I know some time back when they added the in call menu (MCU release 4.3), I had suggested such a thing, don't think it made it anywhere though. :(
As far as changing the logo for conferences, that isn't possible, only on Auto Attendants.