Siva zona.

IMEI information is usualy paired with your IMSI and can be stored by your operator in order to identify you as a user of some device so they can customize their offers for device dependent value added services, or to protect themselfs against abuse of sims embedded in special hardware. Usually a history of IMSI-IMEI pairs is kept and it is posible to trace any change in hardware for a selected sim. Police could requst this info from operators if court so orders, but IMHO it is not a practice for tracing stolen phones. I guess investigation costs much more then a stolen phone, and such events unless they are organized crime, or similar issue, are not interesting to the police.

You would be amazed how many IMEI duplicates one can find in a single network, and I can't imagine all the reasons why this is so, but it renders them useless for unique identification.

If you know how to, and in dilema to change imei or not. Just don't. Your device could be identifed as something else, and change itself can be noticed.