1976 KZ900 Mikuni VM26 - VM28 ??

Question , are you able to run the 73-75 Mikuni VM28 carbs on a 76 originally equipped with VM26's. And if so and effects, or changes that need to be made?
Thanks in advance!

The short answer is Yes,  jetting requirements will be determined by your bike and what modifications have been done to it.

Mike

The bike has the stock airbox and stock reproduction  4 into 4 exhaust .

I'm running VM28's on my 76 900 ltd which had VM26's as stock and noticed the fuel petcock is very close to the top of the carbs.

Well at that point you should use the factory settings. Basically you will have a Z1 engine in a Kz frame. Do you have ‘73 to early ‘74 with a large drain in the bowl, or the later with the small brass drain. Once we know that we can get you stock settings. You need to clean the carbs and document, jet needle and it’s clip position, needle jet size, pilot and main jet, and slide cutaway. Then you can install as necessary to get back to factory settings. 

The early two bolt top vm28sc carbs will perk up the motor over the 26's due to the larger venturi but the early SC carbs are not as good as the later 26/28ss carbs for maintaining carb sync so wouldn't be my first choice unless i had to use them.
All the 903/1015 carbed motors are basically the same engine with differing carbs specs only needed for the different exhaust and airbox design of a particular model.
Fitting the sc carbs to your 76 with airbox and 4x4 should need very little if any changes to the stock jetting apart from some minor tweaking.