You can convert from GPT to MBR and MBR to GPT without data loss (I have tried that) with gdisk in linuxLinux.

Use at your own risk

Run command gdisk /dev/sdx with sdx as per your HDD partition

GPT fdisk (gdisk) version 1.0.1

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


***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format.
THIS OPERATION IS POTENTIALLY DESTRUCTIVE! Exit by typing 'q' if
you don't want to convert your MBR partitions to GPT format!
***************************************************************


Command (? for help): 

MBR to GPT

Enter w to write GPT partition on disk.

Press y to confirm your choice.

GPT to MBR

Enter r to enter in recovery and transformation options.

Enter g to convert GPT to MBR partition.

For Information

You can check table by command p.

Warning: You will lostlose your boot loader (Ex. GRUB)

You can check if your partition is GPT or MBR now gdisk /dev/sdx with sdx as per your HDD partition in Partition table scan(p)