Often times this change has to be made for simple economic reasons; if the old mechanical movement is worn out, the price of a new German made mechanical movement can be very high. Sometimes it’s also just for less work and maintenance. Either way, some considerations have to be made when changing a spring wound or weight driven movement for a battery operated movement. If you think your clock may have any value, keep the original mechanical movement lightly oiled in a plastic bag and store safely.
First remove the clock hands. If you have a second hand it will normally just pull off. The minute hand (the longer one) is usually held in place by a small round nut that needs to be unscrewed,