To Change IMEI number on Android Phone you need to root your phone First.
You Can Use SuperOneClick to root your Android Phone.
1. Step One.
Install terminal Emulator (you can found free in the Market).
Run The Terminal Emulator
3. Step Three
You've just change your Imei Number
command "su" is to granted the SuperUser Access
You can Change Your15digitIMEI with any number you like to be your new IMEI.
On several China Android Phone they have 2 SIM Card to change second IMEI number use this command:
If you change Your IMEI number on "SIM Locked Phone" That Mean You've Just "Unlock It".
if You Have Whatsapp Massanger Installed on your device before you Change The IMEI, That mean you need to Reconfigure your Whatsapp Acount.
I hope that can help you.
Read My Post About HOW To Root Android Phone.
This is revision about last post.