Howto add older macs to Apple Business Manager

The only way to add it would be to contact the original reseller and request that it be submitted for association with your ABM. Please note, if the unit was purchased through retail channels (walk into the Apple Store, from Amazon, from an unassociated ABM reseller), you will not be able to add it as you will not be able to prove proper chain of custody nor will the purchasing channel have access to submit the hardware for ABM association.

However, if you purchased the unit from the same reseller you are purchasing from now (who is presumably enrolling them in ABM at time of purchase), you should be able to ask them to go back to older invoices and enroll prior purchases. If I recall, we've been able to go back to 2012 invoices for backloading.

Ah, but now for the question of why? The 2015 models are capped at Monterey. Apple just announced that Sonoma will be released later this month. Once Sonoma is out, that 2015 will have one year of Monterey updates left before it is out of software maintenance. You may be going through a lot of effort for a machine with limited life left.

Hope this is helpful.

I think the right move here is to replace the 2015 through proper purchasing channels to ensure the new hardware is liked to ABM at time of purchase. This will allow the new device to auto enroll and replace the 2015.