There are quite a few changes, ranging from the fun and/or cosmetic to full re-imaginings of certain items. It’s well worth noting that some legendaries are not changing; the blog post specifically cites the Helm of Command as an example.
Some excellent news is that some of the 1.0.4 legendaries are now item level 63, up from their previous cap of 62.
Some of the new legendary item graphics can be seen in the video below, as well as a few of the new proc effects. Enjoy!