You're going to need someone with experience helping you.
If you are able to isolate the boiler from the rest of the system by closing handvalves, you're ahead of the game. If not, now is an excellent time to add them. Turn off all power to the unit and all sources of water. Otherwise, drain the whole system, add handvalves, and isolation valves on either side of your circulator(s) and anywhere else where you might have to replace a part and don't want a hassle. I like to use valves and unions at the coil.
If you can isolate the boiler, just drain it so the water is below the coil's level.
I would try to remove each coil nut ahead of time. Take one off and put it back on and then go to the next one. You don't want to snap any studs in the middle of your project. I use Never Seize on the studs. From disassembling old boilers, I know that the studs are very hard to break off, but don't push your luck.
Inspect the steel surface under the gasket and on the coil plate. Remove any rust. Don't use sealer and don't overtighten the nuts. The gasket will push out a little and then stop tightening. You can always tighten some more, but when the gasket squishes out, it's done for. Check every year.
Draining a system and refilling with fresh water is bad enough, but it won't kill a boiler like a continuous supply of fresh water being added because of a leak. It's something that has to be done.
The aquastat just loosens with one screw and pulls out. It's not a big deal. There is no loss of water.
Bleed out your system. I never plagued myself on my own systems. I used a garden hose hooked to the boiler drain and forced water, slowly, through one side of each loop and on the other end, where there should be a hose bib, I hooked a garden hose and put the other end of that hose higher than my highest baseboard, on a pole or something, and ran until the bubbles stopped. Do each zone that way, from the lowest to the highest. I never had to re-bleed any lines that way. If the circulator was able to circulate and my air vents were working, I was done.