Just did my B7510. Same thing. Manual says so.
Anyway, you need the Kubota filter wrench proivided with your owner's manual, probably a strap wrench (as my filter wrench wouldn't work on the "double painted" factory filters - and one was severly dented in shipping prior to assembly), a 26mm for the hydro strainer screen (I used an 1-1/16" socket), 14mm for the front drain plug, and a 17mm for the THREE rear drain plugs.
I had no problems when I did my B7500 two years ago, but this time on my B7510, I forgot the one drain plug on the right side behing the screen plug as thye maual says "both sides",but I saw "both plugs" - and there are three, not two on the rear. When I filled up the case with new UDT, it overfilled with the first of the LAST two quarts. I thought it was air-bound, so I removed the front plug, bleeded a fair bit of air, and the level went down, but still ran out the dipstick tube. Ultimately, I re-read the manual and realized the one plug I missed, drained some from that plug to "remove" any settled filings or dirt on that side of the rear-end, and drained the last bit of overfilled fluid out of the rear center plug. I figured that after replacing 98% of the old fluid with new fluid, cleaning the hydro screen, and draining all points of the transmission, I should be good to go. I had mostly mowing time on the tractor, and know it was not abused by me, so I will do my "50 Hour" service again next year, if I feel so inclined. After all, the next required service fluid/filter change is at 300 hours. That two quarts not drained shouldn't make a hill of beans. The filters and fluid (Super UDT) are too expensive to do it again this year with the way I baby the tractor anyway.
Good luck. Be prepared for a loooooooong drain time for the front drain plug. Two hours and mine finally stopped. I tried everything. Anticipation!