I find working with repo as difficult as working with Android source code. We faced a lot of trouble while working with different versions of Android Source code. So I googled for repo command help but didnt find much help. So I experimented a bit and found a good way of switching branch.</br></br>

1. git reset command for removing changes that you have made

$ repo forall -c git reset --hard

1. then initialize repo with new branch.Suppose you have checked out version 4.0.4_r1.2 and you want to revert to 4.0.1_r1 (which was my case actually :P) then

$ repo init -u https://android.googlesource.com/platform/manifest -b android-4.0.1_r1

1. Sync repo

$ repo sync

1. It’s applicable for all combinations

And magic happens :).
Hope you will find this useful. Cheers and happy coding m/.