I cannot download purchased content

You need to contact support and change all the information

Is this the only content you cannot purchase? Are you able to download free content and content you have already purchased before?

If you read the warnings before the migration it says this:

Previously purchased or downloaded content (for example, videos, games, and add-ons) cannot be downloaded again after migration. Previously purchased content is not refundable, and there is no guarantee that it will work after migration.

You cannot purchase Xbox 360 content from the Xbox One store.

For digital download content, manage the game and they should be there ready to be installed. Make sure to be using the account that purchased them.

You can check your account at
https://billing.microsoft.com for any errors or to see if there are any outstanding balances. If you're getting some kind of error code when you try to make a purchase you can post it to see if it helps narrow down the issue. But calling is the only way
to get in touch with the billing department.