Can IMEI number be changed?

I wonder if IMEI number of phone can really be changed. I've been searching in the Internet for a while, but did not find a satisfying answer, because all of them differ from each other. I am going to summarize what I've found by now: 1. IMEI cannot be changed because it is stored in ROM memory, 2. IMEI can be changed using certain apps that are able to do this, 3. Such apps only change IMEI number that is displayed on the screen, but the real one stay untouched (supposed that it is true - how can I check if displayed IMEI is real or fake and how to check then the real one ?) 4. You can change IMEI via Service/Engineer Mode that can be accessed by typing secret code (I tried this and I couldn't change there anything - there were only infos) So as you can see, these answers are completely different. Could anyone refer to it?