iPad to iPhone scaling - creating a universal app?

maybe silly questions but I am new to this…
I have created a simple iPad app and I have it running ok via Xcode in the simulator - the project makes use of fonts and therefore ‘fontSize’ (specific dimension variables…) and fullscreen images as information screens using ‘sprite’ to display them. I realised that I needed to upscale the dpi of images so they looked good on an iPad 3 and 4 (I am using iPad2) but now I am wondering about how to make the whole project ‘Universal’ - iPhone compatible and if there is a quicker way to do this?
I guess not and I also guess I am not fully understanding how to create a ‘Universal’ app.
I am thinking I need to recode portions of the project to make it correctly visible on the existing different iPhone formats but then I am basically creating entirely new apps… so is there a nifty way to rescale the project or do I need to create different versions and then submit all versions individually to the appstore?
Any advice greatly appreciated - thanks!

Read this tutorial from here: Interlude 13 - Pixels vs Points in iOS

ok thankyou Georgian! The tutorial previous to this was also very helpful - for anyone else with similar questions I would start here >