Changing IMEI number is problematic in most countries, because it is in conflict with the law. It is the same as if you wanted to change the serial number of your car. If they catch you doing this you can go to jail.
With older phones it was easier than with new phones because new phones have better security and also one time programable chips. As you can read on some forums, some people change the phones motherboard instead of programing chips.
For changing IMEI you also need professional equipment and a lot of knowledge.
Why to change IMEI number then?
There are different reasons for IMEI changing:
- to delete tracks about stolen or lost phone (this reason is used mostly )
- to delete tracks about phone manufacturer and model
- for researching
- other reasons
How can manufacturers prevent IMEI changing?
Actualy there is no 100% security just as all other problematic things on the world. Manufacturers can do their best but still you can change chip and reprogram it if you have the same knowledge as manufacturers employees.
How to check if phone is stolen or lost?
You can call mobile operators in your country and ask for IMEI number or you can also check the database of Stolen, lost and found mobile phones.
Tips when buying used mobile phone
You can also read some tips when buying used mobile phone on this link