Does changing mobile mother board changes IMEI no ?

Where is the IMEI stored in cell phone? this was the question asked by one of my friend who just got replaced his handset with a new motherboard! his phone was water logged and his phone stopped working repair guys at the service center said that mother board has to been damaged and need to be replaced, but he had a doubt whether they really replaced the mother board or just repaired the phone and gave him back with same mother board.

Though i suggested him to check with his old IMEI number by dialing * # 06 # he said asked me that whether it is not possible to change IMEI no with any software and change it to new one ? though after search i came to know that there are few special softwares that does the work like Ziphone utility i really don't know whether it is true and indeed we can really generate and change the phones IMEI if that was the case i guess many people who have chinese handsets would have done for themselves.

if any of you guys reading this know more about this do enlighten us with your feedback