Hi nugun,

The resource below explains how to change your Apple ID, and what to do after you make a change.  Is this the process you used?

Change your Apple ID

Change your Apple ID

Follow these steps to change the email address that you use as your Apple ID.

1. Sign out of iCloud, the iTunes & App store, FaceTime, Find My Friends, Find My iPhone, and iMessage on each device that uses your Apple ID.
2. Sign in to your Apple ID account page.
3. In the Account section, click Edit > Change Email Address.
4. Enter the email address you want to use as your Apple ID. A verification code will be sent to your new email address.
5. Enter the verification code from the email into the fields provided and start using your updated Apple ID. Didn't get the email?

You can change your Apple ID to any other email address you control, as long as it's not already in use as an Apple ID. If your email address ends with @icloud.com, @me.com, or @mac.com, you can't change your Apple ID and you won't see Change Email Address on your account page.

Then follow these steps on your iPad.  (I've included some key information here for you.  Click on the link for complete details.)

What to do after you change your Apple ID email address or password

After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services that you're using.

After you change your Apple ID or password, use the alphabetical list below to make sure that each store or service is using your updated sign-in information. You can use these steps to sign in on your iPhone, iPad, iPod touch, Mac, PC, and Apple TV.

Apple Online Store

Go to Settings > iTunes & App Store.

• If you're signed out, enter your updated Apple ID and password.
• If you're signed in with your previous Apple ID, tap your Apple ID, then tap Sign Out. Then enter your updated Apple ID and password.

iCloud

Go to Settings > iCloud.

• If you're signed out, enter your updated Apple ID and password. The data in iCloud will automatically appear on your device after you sign in.
• If you're still signed in with your previous Apple ID: 
   
  1. Scroll down and tap Sign Out, then tap Sign Out to confirm. If you're using iOS 7 or earlier, tap Delete Account, then tap Delete to confirm. You might need to enter your password. Your data will remain in iCloud and will be updated on your device when you sign in to iCloud again.
   
  2. Enter your updated Apple ID and password. 

If you updated your Apple ID email address before signing out of iCloud, and your device repeatedly asks for the password to your previous Apple ID, you can temporarily change your Apple ID to sign in.

 

FaceTime

Go to Settings > FaceTime.

• If you're signed out on your iPhone or iPod touch, tap "Use your Apple ID for FaceTime" and enter your updated Apple ID and password. On your iPad, just enter your updated Apple ID and password.
• If you're signed in with your previous Apple ID, tap your Apple ID, then tap Sign Out. On your iPhone or iPod touch, tap "Use your Apple ID for FaceTime" and enter your updated Apple ID and password to sign in. On your iPad, just enter your updated Apple ID and password.

Find My iPhone

Open the Find My iPhone app on your iPhone, iPad, or iPod touch:

• If you're signed out, enter your updated Apple ID and password.
• If you're signed in with your previous Apple ID, tap Sign Out. Then tap Sign Out again when asked to confirm, and enter your updated Apple ID and password.

Game Center

Update your Apple ID and password on your iPhone, iPad, iPod touch, Mac, and Apple TV (4th generation).

Go to Settings > Game Center.

• If you're signed out, enter your updated Apple ID and password.
• If you're signed in with your previous Apple ID, tap your Apple ID and tap Sign Out. Then enter your updated Apple ID and password.

Messages

After you update your Apple ID, your friends will receive new messages from your updated Apple ID. Old messages aren't removed.

Go to Settings > Messages, then tap Send & Receive.

• If you're signed out on your iPhone or iPod touch, tap "Use your Apple ID for iMessage" then enter your updated Apple ID and password. On your iPad, just enter your updated Apple ID or password.
• If you're signed in with your previous Apple ID, tap your Apple ID, then tap Sign Out. On your iPhone or iPod touch, tap "Use your Apple ID for iMessage" and enter your updated Apple ID or password. On your iPad, just enter your updated Apple ID and password.

Take care,