To make the existing partition 1 an EFI System partition, the command is:
select part 1
set id=c12a7328-f81f-11d2-ba4b-00a0c93ec93b override
Code:
DISKPART> help set id
Changes the partition type field for the partition with focus.
Syntax: SET ID={<BYTE> | <GUID>} [OVERRIDE] [NOERR]
ID={<BYTE> | <GUID>}
Specifies the new partition type.
For master boot record (MBR) disks, you can specify a partition
type byte, in hexadecimal form, for the partition. Any
partition type byte can be specified with this parameter except
for type 0x42 (LDM partition). Note that the leading '0x' is
omitted when specifying the hexadecimal partition type.
For GUID partition table (GPT) disks you can specify a
partition type GUID for the partition. Recognized GUIDs
include:
EFI System partition:
c12a7328-f81f-11d2-ba4b-00a0c93ec93b
Basic data partition:
ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
Any partition type GUID can be specified with this parameter
except for the following:
Microsoft Reserved partition:
e3c9e316-0b5c-4db8-817d-f92df00215ae
LDM Metadata partition on a dynamic disk:
5808c8aa-7e8f-42e0-85d2-e1e90434cfb3
LDM Data partition on a dynamic disk:
af9b60a0-1431-4f62-bc68-3311714a69ad
Cluster Metadata partition:
db97dba9-0840-4bae-97f0-ffb9a327c7e1
Other than the limitations mentioned, DiskPart otherwise does
not check the partition type for validity except to ensure that
it is a byte in hexadecimal form or a GUID.
OVERRIDE Enables DiskPart to force the file system on the volume to
dismount before changing the partition type. When changing
the partition type, DiskPart will attempt to lock and dismount
the file system on the volume. If this parameter is not specified,
and the call to lock the file system fails, (because some other
application has an open handle to the volume), the entire
operation will fail. When this parameter is specified, the
dismount is forced even if the call to lock the file system
fails. When a file system is dismounted, all opened handles to
the volume will become invalid.
NOERR For scripting only. When an error is encountered, DiskPart
continues to process commands as if the error did not occur.
Without the NOERR parameter, an error causes DiskPart to exit
with an error code.
Intended for Original Equipment Manufacturer (OEM) use only.
A partition must be selected for this operation to succeed.
Caution:
Changing partition type fields with this parameter might cause your
computer to fail or be unable to start up. Unless you are an OEM or an
IT professional experienced with GPT disks, do not change partition
type fields on GPT disks using this parameter. Instead, always use the
CREATE PARTITION EFI command to create EFI System partitions, the
CREATE PARTITION MSR command to create Microsoft Reserved partitions,
and the CREATE PARTITION PRIMARY command without the ID parameter to
create primary partitions on GPT disks.
This command does not work on dynamic disks nor on Microsoft Reserved
partitions.
Example:
SET ID=07 OVERRIDE
SET ID=ebd0a0a2-b9e5-4433-87c0-68b6b72699c7