I remember when jQuery 1.4 came out, there was a upgrade link and text where it said that there where stuff that was changed, and stuff we had to rewrite for it to work in 1
The differences between 1.3.2 are listed here: http://jquery14.com/day-01/jquery-14
And the release notes for 1.5: http://blog.jquery.com/2011/01/31/jquery-15-released/
The 1.4 link has a list of breaking changes, including a link to this plugin to counteract the breaking changes.
And now 1.6 as well: http://blog.jquery.com/2011/05/03/jquery-16-released/