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just remember to leave the current heater unplugged for 30 minutes before you remove it, and once you put the new on in the tank wait another 30 minutes before you plug it in. in order to prevent your tank from cooling too much, i think you could place the new heater(unplugged) into your tank and unplug the current one, and let the two sit in there unplugged for 30 minutes. this time allows to heaters to cool off/warm up to prevent them from cracking.
Or, get a visitherm stealth (no glass there), put it in, plug it in, make sure it doesnt' go beyond your desired temp (there's a dial w/ temp settings on it, but I'd trust a thermometer more than the dial of any heater).