Change partition table

[Avidanborisov]

how can I change the partition table to MBR from GUID?

[oldfred]

Make sure you have good backups. You will not be able to boot windows unless you have efi not BIOS. Any partition changes has some risk.

I would also make a backup of partition table and save to another device.
Backup partition table to text file
sudo sfdisk -d /dev/sda > PT.txt

Download the newest copy of gdisk:
GPT fdisk Tutorial -srs5694 in forums
http://ubuntuforums.org/showthread.php?t=1439794
http://www.rodsbooks.com/gdisk/


Conversion - See several posts by srs5694:
http://ubuntuforums.org/showthread.php?t=1454252

You will need another small partition for grub booting:
If you have gpt you need another small (8-32mb) partition for grub. Are you using efi or BIOS compatible mode?
http://grub.enbug.org/BIOS_Boot_Partition

srs5694 is pretty active in this forum. If you add gpt to your title he will probably add some additional info: