switching from a locked account to a non-locked account

Those long strung out sentences make it very hard to follow what you are asking.

You will have to ask Apple to reset the iPhone. This is an online-only service. Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Read the requirements and steps for an Activation Lock support request at: https://al-support.apple.com/#/additional-support Apple says you need to have "proof of purchase documentation” and “proof of ownership documentation”, but does not detail what currently constitutes proof other than it “must include the product serial number, IMEI or MEID.” User reports on this community indicate that typically Apple will ONLY accept an original receipt issued by Apple or an Apple Authorized Reseller. Presumably this is the only kind Apple can authenticate against fraud since Apple is not involved in third party sales. Apple recommends that you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support

If Apple resets the device this will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

The old account is set on the iPhone in the Find My setting. In order to remove this you need the information used to set Find My and probably the account itself has to be active. Since the account is no longer active then an activation lock removal request is the only way to remove it. Until that time you will not be able to change certain settings on the iPhone. It will ask for the old account information. If Apple will not remove the lock setting then you may be able to continue to use the iPhone as it is but at some stage it may become impossible and you will have to buy a new device.