Different Apple ID's
I have an Apple ID for login into my devices and another Apple ID for subscriptions, how can I merge or change my subscriptions to my original ID without losing my purchases?
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You can't merge the accounts or change your subscriptions to accomplish what you want, but you can set up family sharing between the IDs to share purchases made with them. See:
Family Sharing https://support.apple.com/en-us/HT201060
Set up Family Sharing https://support.apple.com/en-us/HT201088