I follow my B2650 manual's maintenance schedule.
So replaced the HST and sump filters, and engine oil & filter @ 50 hours service , then only the HST filter at 200, along with the engine oil and filter, as per maintenance chart.
I got set up at 50 hours with new hst filter lubed and ready, and a quick spin off of the old then quickly spin on the new sump filter. No jacking the tractor, so vacuuming, just act quickly, with the verified correct filter lubed ready to go. I lost maybe 1 litre of hydraulic oil before the new filter got tightened on. There was no sign of metal shaving or particles on or in the old filter, which I was advised to inspect when I changed it. Take pics of any such signs so if there's any warranty issue later there's a possible clue.
The sump filter I did replace again along with the Premium UDT oil at 400 hours.
Why would Kubota say no udt fluid change until 400 hours if there was any reasonable chance that leaving it for 400 hours, with required filter changes, would be at all risky for the tractor's hydraulic system?
And just my opinion, backed by remarks from my Kubota service manager, is that the specs Kubota publishes for their tractor maintenance schedule has pretty much a 100% fudge factor. In other words, all the fluids and filters could run twice as long as specified under average operating conditions and still effectively run and protect the machine without significant power, functionality, or wear longevity losses.
Just my opinion. No judgement of those that see a need to do the "400 hour" or full change of fluids and filters at 50 hours. That's obviously your prerogative and won't hurt anything but your wallet and add likely good oil to the recycle pail.
Again, in IMO, and Kubota's, unnecessary.