If your Apple ID is already an iCloud account, you can’t change it to a different iCloud account, as far as I am aware.I have gone through a similar thing a few years back, when I wanted to change my Apple ID to a new iCloud Account. You simply cannot, iCloud is an Apple ID, so changing your current Apple ID to an iCloud account, is the same as changing an Apple ID for another Apple ID. I simply had to bite the bullet, and accept that purchases made on my other account were gone.