how to change Xbox gamertag in region that not available?

"How to change your Xbox gamertag"

Here's a link to a website that literally checks if a GamerTag is even available on Xbox in the first place!

"How to tell if a xbox gamertag is available

Change your gamertag through a web browser
Here’s how:

1. Go to the Change gamertag page at
   account.xbox.com.
2. Sign in using the Microsoft account email address and password associated with your gamertag.
   
3. Enter your new gamertag, and then select Check Availability.
   
4. Once you find an available gamertag, click Claim it.
   
   Note If your gamertag was created for you when you initially signed up for Xbox (meaning you did not choose it yourself), you can change it one time for free. Additional Xbox gamertag changes cost extra.
   
   If you created your own gamertag at signup and want to change it, you will be charged (fee varies by region and currency).
   

You can also go to Microsoft Account page to add and manage your payment options. This could be related to he region your Microsoft account is associated with.

How to update your Microsoft account if you’re moving to a new country or region

How to change the region? It is not available to change.

Change your gamertag through a web browser

Here’s how:

1. Go to the Change gamertag page at account.xbox.com.
2. Sign in using the Microsoft account email address and password associated with your gamertag.
3. Enter your new gamertag, and then select Check Availability.
4. Once you find an available gamertag, click Claim it.
   
   Note If your gamertag was created for you when you initially signed up for Xbox (meaning you did not choose it yourself), you can change it one time for free. Additional Xbox gamertag changes cost extra.
   
   If you created your own gamertag at signup and want to change it, you will be charged (fee varies by region and currency).

• A gamertag can contain a maximum of 15 characters.

In changing a Gamertag the physical location, account region, and payment method need to all match. Currently, your region is not yet supported on Xbox Live which may be causing the issue since your Xbox profile region can't be changed to the Philippines since it's not yet an available option.

https://live.xbox.com/AccountMigration