As you are changing the hydraulic and HST filters you will catch some of the hydraulic fluid, this will allow you to inspect the Super UDT2 fluid and then you can make an informed decision. If you don't mind the cost I would replace the fluid after the initial 50 hours, you need approximately 6.2 gallons ($125 - $130). From this point you can follow the general fluid change recommendations.
I often change my fluid earlier than the recommended hours due to the low hours I put on the tractor each year. Typically I will change it at 200 hours as it may be 5 years since the prior change.
As commented above the front axle has no filter, my personal opinion is to change the front axle fluid no matter what as it only takes just over a gallon.