Draining the reservoir only is a waste of good oil because the oil in the cylinders and lines will contaminate it as soon as you start the tractor.
I get your comment.... but would ask (first the background)
I've got a JCB 1550b. Had a hose leak from the pump, Replaced hose and ended up draining tank to put new fluid in (not done yet). I'm waiting on a new screener that goes inside the tank.... then I put back together and start adding oil in. Tank is essentially bone dry because I cleaned out as much as I could through the access panel on both sides.
If I have oil in the tank, I get there will be oil in the lines BUT.... I'm looking to remove (one at a time) a line on say, the backhoe boom.... fill it (with new oil) BUT, the oil already in there would be coming out of the hose I removed and dumped/pumped into a bucket for disposal. Also, since most of the cylinders are at full extension or full retraction, as I operate it from say (full retraction to full extension) I can get the old oil out of one side.... if I don't mind wasting some new oil, I can then swap open hoses and as cylinder goes opposite direction get rid of the old (plus newly added new) oil...
Go through the cylinders and stablizers, seems to me you could put a serious dent in the old fluid, no??
It will never be a clean flush unless there is some machine or you do it another time....
I don't know how great the idea is but this is what I'm looking to do in my feeble attempt to extract as much of the old oil as I can. I know it won't be perfect but, will be much better than before!!