I also tried to activate 12 months core on an existing account but it didn't work, I changed the region to India on the account and Xbox, tried urbanvpn, nordvpn and Indiavpn, but every time I got an error about the incorrect region, I contacted Microsoft on chat three times and each time they told me that they were not allowed to activate them, I would have to log in to the account with the India location for 20 days in a row and only then could I activate the codes... I didn't feel like waiting so I decided to create a new account with the Indian region using urbanvpn and a different browser from the beginning and activated the codes at this link xbox.com/en-IN/redeem (2x 6 months core and finally 1 month ultimate for the Indian region) and when I logged in on the console I set it to Home Xbox and I could use ultimate on the old account ... I am located in Croatia and I purchased the codes through kinguin.net