I changed the Hydro Fluid in my 1864 at the end of last season because I didn't know if it had ever been done. There were only 135 hours on the machine.
Luckily, it has the older heavy-duty Sustrand, which uses the inexpensive Hydro-Plus fluid, not this new expensive funky synthetic stuff
If yours is anything like mine, you need a BIG drain pan! Mine holds seven quarts.
Also, if your filter is positioned anything like mine, it is NOT easy to work with. Make sure you have a couple of filter wrenches, including a strap wrench, handy as you may have to try different one's.
Again, if yours is like mine, re-filling will be painful as the tube is VERY small (a funnel won't even fit into it). You almost have to dribble the fluid back in and it takes forever.
Don't overfill. I stopped at 6.5 quarts and started it up and ran it around the street until everything was nice and toasty, and then checked it. It was right on the full mark.
If your drain pan is one of the enclosed one's, don't forget to remove the vent plug when you drain the hydro. At the rate that fluid comes out it will air-pocket up on you in a heartbeat and overflow over the edges of the drain pan instead of going down into the hole in the middle!
-Larry