I have some updates (minor achievements, etc) which I would like to send to Harvard University. Would it be appropriate to send a list of achievements in an update letter to Harvard if I did not apply there early action?
Thanks!
You answered your own question, and it has nothing to do with applying RD vs. SCEA:
My thoughts:
Harvard will assume that most applicants will continue to receive honors and awards after the application has been submitted. You can certainly update them with significant updates, which in my mind is something that makes the regional/national papers outside of the free local weekly. But to update them because you receive a varsity letter or the Most Improved Player Award? No. They have more than enough information to make their decisions; blinding them with paper will not improve your chances, IMO.
I doubt it will hurt or improve your chances as, by your own statement, they are minor achievements. Generally, updates are best saved for if you happen to win a major prize (regional or national-level, etc.), publish research, publish a book, win a major national-level or higher competition, or something equivalent after your application date.
Note, a list of minor achievements is not equivalent to one major prize - many Harvard students have endless lists of minor achievements that they never end up submitting.