Apple, as noted by Stephen Hackett of 512 Pixels, has published a support document, on how to migrate purchases from one Apple Account to another Apple Account.
You can do this if an Apple Account is only used for making purchases, those purchases can be migrated to a primary Apple Account to consolidate them. Also note that this feature isn’t available to users in the European Union, United Kingdom, or India.
From the Apple Support doc: You can choose to migrate apps, music, and other content you’ve purchased from Apple on a secondary Apple Account to a primary Apple Account. The secondary Apple Account might be an account that’s only used for purchases. You’ll need to know the primary email address or phone number and password for both accounts, and neither account should be shared with anyone else. Learn more about migrating purchases between accounts.
- At the time of migration, the Apple Account signed in for use with iCloud and most features on your iPhone or iPad will be referred to as the primary Apple Account.
- At the time of migration, the Apple Account signed in just for use with Media & Purchases will be referred to as the secondary Apple Account.
