I am fairly new to the AR and was wondering if there are other uppers in different calibers that could be switch onto the stock AR15 lower?

Yes, in the AR15 platform you can go from 22lr all the way to 458 SOCOM or 500 Beowulf. The AR10 is even more versatile

Is there a list of upper receiver calibers that will work on the AR lower without modifications?

Generally any upper works, sometimes a new mag will be required. Info will be supplied by the manufacturer of the upper.