I just replaced the block heater on my BX24 and thought this might be a good place to post my project since the above information was helpful to me.
I started with a cool engine (and a heated garage!). I removed the cowling to gain access to the engine, but found that I also had to remove my quick hitch and, of course, my snow blower in order to get the cowling off. Other posts here helped me figure out how to drain the coolant. I just used the block drain and saved the coolant since I wasn't in need of a flush/change.
As noted the heater is pressed into the block, so it has to be pried/twisted out. The dealer told me to use water pump pliers, but I found it hard to get a good grab on the heater and get any leverage. I was quickly feeling like this quick project (during a three day snow storm) was becoming a bigger deal than I had bargained for and also a "must complete today" project.
While pondering how to get this thing off the block, I thought a pipe wrench might be a better tool and then immediately thought that if I added a pipe to the heater (which is threaded on the exposed end) it would make it super easy to turn. So I purchased a 3/4" coupler, 12" x 3/4" nipple and just for good measure, a 3/4" cap. I mated the pipe and coupler to the heater and turned everything clockwise to tighten the threads and start the twist-out.
As soon as the threads were all tight, the heater twisted and the pipe provided a nice handle and lateral leverage for popping the heater out. Bingo...that was easy.
I installed the new heater as described above except after initially tapping the heater in place with a 1" socket to protect the threads, I removed the socket and installed the pipe again. This time I added the cap on the end and I tapped on the capped pipe to securely seat the heater into the block. I found this was easier than hammering in the close quarters near the engine.
By initially seating the heater with the socket, I was able to make sure it was properly oriented and partially secure, so threading the pipe on did not screw up the orientation of the heater.
Many thanks to other contributors for making my project much easier.