How to remove old seacocks and thru-hulls
There are two main methods and both of them are approaching the problem from the outside of the hull. Usually it's impossible to take the seacock assemblies apart from the inside due to the bad access and the huge force and large tools needed for breaking the thread-locks and all the old rusty nuts.
From outside what you can do is to separate the thru-hull's flange from the threaded part, then you can just pull the whole thing in. One way is to hammer in a wooden plug, then drill into the hole with a slightly (1-2mm) larger hole-saw than the thru-hull hole (which is the inner part of the thru-hull's threaded pipe). I tried this approach first, but my wooden plug kept rotating and falling out, so I opted for the second approach instead. Take an angle grinder with a metal disk and grind away the flange of the thru-hull. You need to be careful and do it with small motions not to damage the hull, but it's doable. This was the first time for me and I managed to get away with a few lighter scratches in the antifouling . Always wear a mask and a good eye protection, small metal pieces will fly all over. And if your hand starts to get tired, take a break! Otherwise the chances for making a mistake will increase.
Grinding away one thru-hull can take a few minutes, depending on it's size. Once all the flanges were gone and the hoses were disconnected from the seacocks, it was possible to push the old assemblies in. If they are still stuck, you can hammer them in from outside with a larger wooden plug or something similar. But most of them were quite easy to pull out.
I couldn't get one of the hoses off, so I ended up cutting it with a multitool. If you're planning to do a seacock replacement, I suggest checking out the SailingBritaly youtube channel, where Chris recorded a 1 hour long video documenting the whole process. That's where I got the courage from :)
Shiny new Dezincification Resistant (DZR) brass thru hulls, ball valves and hose fittings finally arrived, can't wait to fit them.