You cant boot old OS installations after changing to AHCI…it can but it may be difficult to change it manually for an inexperienced user.
Do a clean OS install if your changing the mode to AHCI for HDD/SSD or NVMe boot, other than that stay as before in IDE mode.
You already bought the NVMe adapter so when changing the system to NVMe boot, you clearly going to do this. I do not advise any OS cloning.
EDIT: The BOSD are port driver issues and i dont know what driver you used, the system has to boot with the default MS OOB drivers for SATA, im sure you dont want to lose time on old OS…
Do please read careful the guide, when decided to migrate the system to NVMe boot.