Converting from MBR to GPT should only be done with basic data, such as external HDDs. If Windows was installed in Legacy MBR mode, it will have the bootloader partition as System Reserved set as active. If Windows was installed in UEFI GPT mode, it will have the bootloader partition in an EFI partition with a GUID identifier of C12A7328-F81F-11D2-BA4B-00A0C93EC93B.
If the Legacy bootloader partition is on a disk converted to GPT, it will not be set as active, as GPT does not support setting partitions as active. If the EFI bootloader partition is on a disk converted to MBR, it will not have the correct GUID identifier. The System Reserved partition can't just be converted to an EFI partition with the correct GUID, and then rebuilt with EFI bootloader files, because a UEFI installation of Windows has the System Reserved along with a separate EFI partition. Adding another partition alongside Reserved would require shrinking the large Primary from the beginning. Switching Windows between MBR and GPT is best done by reformatting and reinstalling.
The BIOS should be switched to Legacy boot mode if a system image backup of Windows 10 is to be restored. If Windows 10 needs to be in UEFI mode, then it'll need to be reinstalled independent from the backup. If the backup is of Windows 11, no conversion is necessary, and only a new recovery DVD is needed.