2.4 Series is NOT compatitable with 2.3xx !!!! I don't think old customer will select it. All module and template need be new!! You will lost many customer!!!
I've already converted all addons I use to work with 2.4, it was too easy. I also created a new theme. Took no time at all.
The good thing is, once 2.4 is released all I need to do is upload my theme, upload my addons and activate them, and BOOM! I'm ready to go in one afternoon without having to touch core code.