On my HST Kubotas, and I suspect Kiotis will behave the same way, you can replace hydraulic and HST filters without draining the fluid. But you may need a couple tricks. Cap of any breathers (I use a nail to plug the breather tube on the transmission case on Kubotas). Some people also pull a vacuum on the fill port on the transmission with a shop vac, but I have never found that necessary. I also park the tractor on an incline or blocks/ramps so that the filter is "uphill".
Doing that procedure correctly, I have never lost more than 1/2 quart of fluid when changing both filters, and most times it's just whatever fluid was in the filters. You can top off to replenish anything that was lost, so have a couple quart bottles around. Last time I forgot to plug the breather and lost about 3/4 quart before I realized what was happening. Still not a whole lot.
You can evaluate the situation by looking at the filters when they come off. Normally the magnetic ring is there, and either gets reused or you get a new one with the new filter. Look at the old ring to see how much metal is on it, and the size of the particles (should be a fine gray fuzz/sludge, but there may be some small chips). See if any larger chips are stuck in the filter ports and ports on the filter mount.
I remember someone on here posted pictures a few years ago with a handful of metal shavings jammed around the filter ports. That was abnormal, and would be a reason for concern. I believe he got in touch with Kubota on that one and talked with engineers. Seriously, it looked like the nest of shavings I see in the bottom of a lathe tray after turning a part!! It was scary. Looked like the transmission case was not properly cleaned out when it was machined during manufacturing.