Hello
When you connect two drives with the same letter to a Windows system, Windows will automatically assign a different drive letter to one of the drives to resolve the conflict. Generally, the drive that is forced to change is the one that is recognized last by the system. In your case, if you plug in the SSD that was previously the boot drive after the new system is up and running, it is likely that the SSD will be the one to receive a new drive letter.
To avoid any issues, you can manually change the drive letter of the old SSD before connecting it to the new computer. You can do this by going to Disk Management in Windows, right-clicking on the drive, and selecting 'Change Drive Letter and Paths'. From there, you can assign a new letter that doesn't conflict with any other drives in your system.
It's also worth noting that if you have any programs or references that rely on the old SSD's drive letter, you may need to update those references to the new drive letter to ensure everything works correctly.