When you start using Windows Phone 7/8, you will need to have and setup a Microsoft Account or Windows Live ID. The first ID you use will be its default ID, and your Phone will be registered against this Microsoft Account ID.
You will require this ID to access features like Marketplace, Xbox LIVE and Zune Pass from your Windows Phone.
But what if you decide to change your default mail Microsoft Account ?
Well there is no way to change this ID, and the only way to do it is to go for a Reset! After a reset, you can use the other ID you want to. The other additional email accounts can ofcourse be removed without needing to reset the phone.
To Reset your Windows Phone, flick and go the App list page and scroll down to Settings. Open it and tap on About, which may appear towards the end of the list. Here you will see the button: reset your phone.
Tap it and your Windows Phone will reset itself back to factory settings!
Do note that resetting your Windows phone back to the original factory settings will erase all personal data, apps photos, videos, music and other content and it may not be possible for you to transfer purchased content to a different Windows Live ID.