For those folks that want to eventually have optional RPM diffs for upgrades, please do not continue the discussion here. Instead if you feel so strongly that it is technically good and without drawbacks, then implement everything needed and provide a test repository and tools at your project site. Only after you feel your project page, tools, and repository are PERFECT, then announce here for testing and comments.
I would suggest that you to research the problem that was discussed by Debian and Red Hat for at least the past 5 years. Many smart people have looked at this problem. Try not to repeat the same mistakes and learn from their prior discussions.
Do not discuss it here because generally the elders are totally not convinced that it is a good thing to do. I personally think it is possible and good to do as an OPTIONAL thing in cases where the diff is below a certain % of the total package size, like 10% for example. However I feel that we have more important things to work on that are higher priority, like Fedora Project's infrastructure, so I personally wont put any effort into this for at least a year or more.
Warren