I don’t think you can just switch it. You probably have to do your own widget. The client side of the ComboBox seems to be VListSelect, so that’s the one to extend. I’m having a hard time interpreting the component but TooltipListBox seems to be the component that is shown when you click the arrow down button, showing the options. Basically, you just have to replace that one component inside ListSelect. I’m not sure if there is another component available that you can choose instead of that one, probably not if you need icons and whatnots. One possiblity to use a one-column table that supports normal scrolling, icons etc 
To summarize, the best way to go would probably be to extend AbstractSelect and take model out of ListSelect. Then again, I could be way off.