https://tools.ietf.org/rfcdiff?difftype=--hwdiff&url2=rfc326...
... is the changes from their last draft to the published RFC
Admittedly only very recent work with git-minded people in the WG has a full git history of revisions, earlier work like SIP just has the numbered drafts representing a few months work typically from one draft to the next, but you can go back and see this work and also the mailing lists that drove the changes.
I'm not a SIP expert, but looks to me like the brief extracts I read are improvements. For example that last draft requires people who want to add a new SIP method (like INVITE or CANCEL) to send IANA a copy of the RFC describing their method. Why? What possible purpose could there be when RFCs are public documents? Are they expected to... print it out and mail it? So, the final standard text just says to provide the RFC number.
I've read the SIP RFCs. That's not a hard rule unfortunately. (Unless it really was worse before, but I can't imagine that...)