I have released an app with the 2.2.1 SDK, to realize that I'm pointlessly losing a big percentage of customers still on firmware 2.2 (See this article: http://arstechnica.com/apple/news/2009/03/app-store-lessons-firmware-followup.ars) So I'm thinking of updating the app to the previous SDK, 2.2.
I was wondering if anyone has run into any problem doing this? Is this safe for customers who have already installed the previous version, to install my "downgrading" update?
I know of no reason this would be a problem. Especially with 2.2.1 to 2.2—it was really a very minor bugfix release.
I have changed an app store app from 2.2.1 to 2.2, and I did not encounter any problems when switching.
The rule of thumb is to build and test your app for the lowest version SDK you can (based on features required).
Iphone os version 2.2.1 introduced no new APIs so you should have no problem to downgrading it to 2.2.
In terms of the other SDK versions (i.e 2.0 and 2.1) apple introduced new/updated APIs. If your app takes advantage of these new/updated APIs you should compile it against the lowest version of the iPhone OS that will successfully run your program.
Compiling against the lowest version possible of the iPhone OS will ensure that your app will be able to be run by the max number of people possible.