You can't. You can transfer some data but anything purchased with one ID remains permanently tied to it. You will have to set up Family Sharing which will have its own ramifications in terms of how the payment system is run.
Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380
Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022
Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.
Family Sharing - https://support.apple.com/HT201060
Set up Family Sharing - https://support.apple.com/HT201088
The payment method for Family Sharing is automatic:
1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple ID balance, if any.
2. If a family member does not have enough personal balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. I assume if it cannot bill the primary payment method then it will just work its way down the Organizer's list of payment methods until it finds one it can.
- "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."
Ref:
- Family purchases and payments - https://support.apple.com/HT201079
- How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359
- Check your Apple ID balance - https://support.apple.com/HT202359