IMEI registered under a different name -I am unable to unlock the phone

Note: The replacement phone I bought through Best Buy in Sept 2019 was a "refurbished" phone.

I also have a NEW UNLOCKED Galaxy 23+ arriving tomorrow.  Do I wait to transfer the SIM card to that phone and then switch to PREPAID? 

It doesn’t matter whether you move your sim to the new phone or not but you might not be allowed to activate the Note 8 on prepaid because of that debt. When you switch to prepaid, do it with the new phone.

Will AT&T lock me into another 30-60 days of POSTPAID if I do that?

No. Be aware you can only switch to prepaid in store, preferably a corporate store. Also, since you’ll have an unlocked phone, you don’t have to stay with ATT. You can port your number to a different carrier, if you want.

It's important to be able to use your phone as you intend, Stephen888, especially since you've owned it for a while. Let's look into what you can do to unlock your phone.

First, since the replacement Samsung AT&T Galaxy Note 8 has an outstanding balance then the previous owner will need to pay any remaining balance that's due before the phone can be unlocked. In this instance, you will need to consult the seller about the device.

You can find similar details and more under Personal device requirements when you visit Get info about device unlock eligibility. There is additional information beneath eBay & Non-AT&T Purchases at AT&T Device Unlock Help - Solutions & FAQs.

Next, since you're interested in an AT&T PREPAID account you will need to visit your nearest AT&T Store to convert your phone number from AT&T Postpaid to AT&T PREPAID before slipping your current SIM card into your new unlocked Samsung Galaxy S23 Plus.

The best place to begin the conversion is at an AT&T Corporate Store. Once you've visited our AT&T Store Locator choose FILTER followed by Company Owned Store to get started.

We hope this information helps. Let us know if you need any other assistance with AT&T products and services.

Thank you for reaching out to AT&T Community Forums.

Lar, AT&T Community Specialist

ATTHelp - thank you for the reply.  Unfortunately, I do not know the previous owner of the replacement phone.  I purchased as a "refurbished" phone through BestBuy four years ago.  It is unlikely that this phone will ever be unlocked.  I'll just continue to use it as a camera or backup mini-computer device once I switch my new phone over as you have suggested.  Thank you for telling me that I need to convert my phine number first before inserting the SIM.  I appreciate that tip!

sandblaster - thank you!  Corporate store - noted.  Yes, I purchased the unlocked phone for the very reason to give myself options as you have wisely pointed out.  I may go with ATT prepaid, I may not.  The beauty of an unlocked phone!

Happy to help Stephen888.

Let us know if you need any other assistance with AT&T products and services.

Thank you for reaching out to AT&T Community Forums.

Lar, AT&T Community Specialist