Change Apple ID without losing everything

You can, read carefully and follow the instructions: Change your Apple ID - Apple Support

Note: Do not create a new Apple ID, if you do then you will not be able to access your old apps.

Read the link ckuan provided carefully before doing anything. You can readily change the primary third party email used as your AppleID without altering anything with your Apple online services accounts or purchase history. But to avoid headaches, the instructions in that link need to be understood and followed.