If the theme breaks, it resets to default (wp-content/themes/default/) by, well, default.
Thus far, I haven't seen anything that would break a 2.3 theme specifically, unless there was something deprecated previously (that was then removed) which shouldn't have been in there to begin with in a 2.3 theme.
Essentially, all the basics still work, etc. Now admin side stuff, that's a bit different for sure. Many changes back there.
Of course, the limited stuff I've kept up with in testing nightlies every now and then (2.4, which became 2.5, etc). So there could be something that I've missed. I've only got a dozen or so themes (2.3 versions) that I have in there to look at. But it's a decent sampling.
Honestly, themes look to be the least of worries for this upcoming batch.