Issue with Endpoint Migration via API between two tenants

Hello Ellis Donnelly 

We appreciate your reaching out to the Sophos Community Forum.

I have checked. The API is disabled and only works with machines on core agent 2024.1 or above. However, you can try to perform the re-register process on that device, but you have to access that device physically to do so. 

You could try to re-register the agent after turning off tamper protection.

In Sophos Central, go to Devices.
Click your concerned endpoint/server.
On the SUMMARY page, scroll down, then click View password details under Tamper Protection.
When you select the Show Password box under Tamper Protection Password Details, note the password.
Sign in to your concerned Windows endpoint.
Double-click the Sophos Endpoint on the Taskbar.
Click Admin, sign in, and enter the password you noted earlier.
Click the Admin sign-in button.
Select Settings from the top menu.
Tick the box near the top for Override Sophos Central Policy for up to 4 hours.
Move the slider of Tamper Protection to the left.

After Disabling the Tamper protection from the sophos agent,
Download the new setup file from Sophos Central
Open the command prompt as an administrator and type cd <location of the SophosSetup.exe>
Use "SophosSetup.exe --registeronly" (without quotes) command to re-register the product in individual devices.
Please check it in Sophos Central; the devices will get updated.

If these didn't help you, I suggest you reinstall sophos on the device. 

Also, for uninstalling sophos from the device, use the sophos ZAP tool. Once you uninstall sophos from the device, you can download the latest installer from the central console and install it on the device.  

Please refer to this link for the sophos ZAP tool with step by step process: 

https://support.sophos.com/support/s/article/KB-000038989?language=en_US

Let me know how it goes. 

Regards,