As a new member, my 2 cents worth. For my L3700SU hydrostatic, I took a cautious approach. The manual seems contradictory to what is actually required. In one place, it says to drain the fluid before changing the filters, but it only pulls the front two plugs and doesn't drain the rear gear sections. For the 400 hour service, it says to drain at all four plugs, so that is what I believe is the intent - to replace the fluid at 400 hours.
Now, back to your initial question. I began by cleaning around the smaller hydraulic (lift) filter (to minimize falling dirt and reducing the possibility of getting it on the new filter), placed a clean pan underneath, and slowly loosened the old filter to just see how much fluid came out. Some fluid drained, but probably not much more than the volume of the filter itself. I already had my new filter ready to go on; about half full of new fluid and a good coating on the top flat ring seal. As the draining stream slowed, I completely pulled the old filter off and installed the new one. Very simple and easy change out.
For the tranny suction filter (larger of the two), I did the same thing. However, as the new filter was loosened, a considerable stream of fluid drained out, so I let it drain until the stream was substantially reduced. Like before, I had the new filter about half full and the sealing ring oiled up. For this filter, after about 3 gallons had drained, the stream was small enough that I was confident that I could get the new filter on, so I quickly continued the removal of the old and immediately replaced with a new filter. Again, other than being a little messy, very easy to do.
With both filters tightened per the manual (filter/ring touching, one full rotation with filter wrench), and probably about 3 and a half gallons of fluid drained, I could - for the first time - actually see that the fluid level was low in my side-mounted sight glass. I poured about 3 gallons from my 5-gallon new fluid into a smaller and more easily handled container and returned the fluid level until I could easily see it in the sight glass. I then cranked the tractor, lifting the loader and 3PH and moving in forward and reverse to cycle the fluid and ensure the new filters were also filled, and then added about another 1/2 gallon to bring it back up to just above the middle of the sight glass. Easy change out, with just under half new fluid in the tractor. The old fluid will get strained and re-used elsewhere. Although my fluid was clean enough to re-use, the small expense for replacing it with new is nothing more than piece of mind for me.
