gdisk is the best solution to do so. Its no assurance of holding the data back, so pls keep a copy of data for safe.

Assuming your disk is /dev/sdb

# gdisk /dev/sdb
GPT fdisk (gdisk) version 0.8.6

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): r
Recovery/transformation command (? for help): g                           [to convert GPT to MBR]
MBR command (? for help): t
Partition to change type code: 83
MBR command (? for help): p

** NOTE: Partition numbers do NOT indicate final primary/logical status,
** unlike in most MBR partitioning tools!
** Extended partitions are not displayed, but will be generated as required.
Disk size is 976773168 sectors (465.8 GiB)
MBR disk identifier: 0x00000000
MBR partitions:
                                                      Can Be   Can Be
Number  Boot  Start Sector   End Sector   Status   Logical  Primary   Code
   1                  2048    976773119   primary     Y        Y      0x83

Thus you have changed the GPT type to MBR

Once its done, check with fdisk -l command, you can see the Disklabel type as dos.

NOTE: dos is considered to be MBR type.