Changing the type of an App to Universal App later in App Store?
Yes. 

Xcode has built-in support for targeting specific devices with your app. It's as simple as selecting the target device from a drop-down list. 

This could enable you to create an initial version that targets either iPhone or iPad, and then later switch to Universal and submit it as a new version. The App Store will then automatically recognize that the new version of the app is Universal, and you would need to add additional screenshots, etc. for the new devices.