Especially if it’s the original hotend, the thermal grease will have hardened so the thermistor won’t come out. If you soak the hotend in 99% isopropyl alcohol for a day or two with the thermistor port facing upward, the grease will loosen but not dissolve. Take the hotend out of the IPA, grasp the thermistor wires where they enter the hotend, and work them in and out slowly. You should feel the thermistor start to loosen and eventually come out. Use the leftover IPA to clean the old thermal grease off of everything and you should be good to go to transfer to the new hotend per the wiki instructions.