When a change in utility rates results in a 10% or more increase in the UA, how do O/As compute the new allowance? Do O/As simply apply the % increase to the existing UA?

Yes, O/As would apply the utility rate increase to that component of the UA allowance, e.g., electric rates go up 15% so if the UA for the property comprises both electricity costs and gas costs, O/As would apply the 15% to the electricity component of the UA.