A release candidate (not the final release 3) for Bootstap 3 RC 1 was announced Friday (July 26 2013), so there is not yet any official migration guidance.
There are major changes for Bootstrap 3. There is no backwards compatibility with 2.x, so you can't just replace the files. You can however find some useful guidance here at Bootply:
If you use Glyphicons, you will have to add them from http://glyphicons.getbootstrap.com/ ( icons have been moved to a separate repository.) Glyphicons are back since RC2 (180 glyphs in font format from the Glyphicon Halflings set)
it is almost impossible to migrate from bootstrap 2.3 to 3.0. the version change is very drastic. I tried to update my zend framework 2 project with bootstrap 3.0 and the results was a mess (like trying to open with IE 5).
My advice: old project using bootstrap 2.X let with bootstrap 2.x. New starting project, start with 3.0.
In my opinion such version change was a very big error from bootstrap team. They should have done a gradual update 2.4, 2.5, 2.6 ....
If you are having a big application with a lot of files with a lot of customization, every single tool likely that could not answer to most of the case you are facing.
First, of course, I recommend to try: https://pypi.python.org/pypi/b2tob3/0.4 but this won't answer to my need, I forked this version and adapt my own one to be able to migrate together Bootstrap 2 to 3 and Font Awesome 3 to 4 together, some cases that not treat in b23tob3-v0.4 (original one) are trying to treat in my version.