jQuery.noConflict(). Just slip jQuery into your existing project and try it out – dip your toe in. There’s no obligation. You can always remove it and go back to Prototype.
git-svn. Just start using Git with your existing SVN repository and try it out – dip your toe in. There’s no obligation. You can always go back to raw SVN.
While not the only cool feature these projects possess, the toe-dip factor is undeniably huge. I started with both
git-svn. I bet most converts did, too.