One final tip. Loosen (but don't remove!) the old immersion heater
*before* you drain the cylinder.

Yes, and be aware that failure is quite common. You should have a
contingency plan to replace the cylinder. If you could benefit from a
different system, such as a heat bank etc. then consider this now, so you
don't just replace like with like in a panic when an upgrade could be
beneficial.

Also, ensure that the water supply is shut off and the hose connected when
starting to loosen the immersion. If the cylinder splits, you can then, at
least, start draining down the cylinder rather than waiting for the entire
contents to spill out.

Christian.