MailChannels has a description of the changes between 2821/2822 and 5321/5322. While the new RFCs obsolete the old ones, they are more a clarification than actual changes to the protocols. Dave Crocker had this to say about the new documents.
2821/2822 did a meaningful job of cleaning up usage problems with 821/822. They did not attempt to make substantive changes to the nature, semantics or syntax of either. They attempted — and provided — cleanup only.
The new docs are further cleanups. But much, much smaller than the previous round of effort, although 5321 has massive changes to the raw writing, even to the point of having used a professional editor during the revision phase.
The take home message is there are no major changes but a lot of cleanup and clarification.