720 degree C is below the lower temperature of transformation, so you are not transforming the BBC into FCC. Assuming you are working with a high strength low alloy steel or carbon steel, you are in the stress relief temperature range. However, 720 is playing fast and loose. There is a good chance you could exceed the transition temperature in certain locations. Bad news.

850 degrees C is well above the lower temperature of transition, so there will, at a minimum, be partial transformation of the BCC into FCC. Depending on the alloy system, it may be too low a temperature to achieve full transformation, so you are too low to normalize the steel. Again, not a good place to be.

It is very dependent on the specific alloy and chemistry of the steel (I assume you are working with steel) you are performing the PWHT on and the specific code you are working to. At the very least, you are going from a subcritical PWHT to a supercritical PWHT. That would require a new supporting PQR by many welding standards.

What is specified by the WPS and are you willing to requalify the WPS to demonstrate the required mechanical properties are achieved at the higher temperature. If you are already pressed for time, I doubt you have time to requalify the WPS.

The two hour different in PWHT is a minor time constrain in comparison to the time it took to set up and weld and the cost and time expended to repalce the weld if it doesn't work. Pennies versus dollars.

Al