I have an iPhone app that I have started to turn into a universal app, however the process is not complete and I want to release an update to the iPhone version.
I k
I'd consider restriction to iPhone very bad decision - the iPad is designed to run all iPhone apps in compatibilty mode (except for obvious things like telephony). So you will run into severe complaints.
If it is about the half-done enhancements for the iPad part, then just turn those features off and compile for iPhone (targeted device: only iPhone; maybe tweak the plist file, too).