Bootstrap 3 is very different to Bootstrap 2.3.2. In particular, a lot of users who have built their sites using Customizr and are actively using the underlying Bootstrap grid would have to make significant changes. For example the html I am using on my site—based on the Bootstrap 2.3.2 CSS and which lays out my images—would not work with Bootstrap 3.
I would recommend a different theme to implement Bootstrap 3. Those who want to can then migrate, while brand new users can use the new theme. The large number of users of the current theme can rest assured that their sites will continue to work through the upgrades, because Bootstrap 2.3.2 is a good stable base.
I think there would be a lot of work to rewrite for Bootstrap 3.