Android devices usually requires your a primary gmail account to sync your Android device with the free Google Services like Gtalk, Google Maps, Google Search, Youtube, Picasa. In case you have forgotten the password of your old Gmail account or want to change to your new account, you can re-activate your Android Device with your new Gmail account.
First you need to factory reset your device. Follow the instructions to reset your phone. But make sure you take backups of your texts, contacts, etc.
Goto Settings -> Privacy.
Click on Factory Data Reset.
Read the disclaimer and then tap on Reset Phone.
It will take some time to reset your phone and its done.
Once the factory data reset is done on your phone, your phone is just the like the branded new one and it will be much faster than the previous usage you get. This guide is common for all the Android powered smartphone regardless of the brand and now when you go through the phone setup, it will ask for the Gmail account to continue.
Type in your new Gmail account and you are ready to use that Gmail Account as your primary Gmail account on your phone