Upgrading everything to Ultra R8000 except for crankset which is Shimano Sora I currently am running full Shimano Sora.

The crankset is 34/50T. The cassette is 11-34T 9 speed.

If I upgrade all my components to Ultegra, this being the rear derailleur, cassette (11 speed) and the front derailleur. Will the Sora crankset still work with the Ultegra gears? I don't really want to spend $500 on a new Ultegra crank when the one I have now is fine.

I am purchasing the 11-34T Ultegra groupset.
In short, yes but not well. 

The distance between the chainrings in a Sora crankset is different than the Ultegra crankset. So if you have an Ultegra shifter and front mech, you may experience some issues (chain rub, delayed shifting etc).

So it will work, but not as well as it should. 

An alternative is to get a 105 crankset, or other crankset of similar dimensions (like a Praxis Works for example). You could always set it all up with your Sora crankset and see how it goes. But be prepared to get another, more suitable crankset. Maybe post a different question that asks which cheaper cranksets will work with R8000.