HD accidentally reformatted to MBR, how to get back to GUID?
As your Partition starts on sector 1, there is no space for a GUID partition table, which requires 2 sectors between the MBR (sector 0) and the start of the first partition as a minimum. You could try to move the partition with a specialist tool (the tools that come with OSX only let you change the size of a partition, not the starting position). 

However, as you're OK with re-installing the OS, I suggest you make sure everything is backed up via Time Machine, then boot from the OSX installation stick. Open up Disk Utility before starting the installation, select the whole drive and open the "partition" tab. Repartition it from scratch by selecting "1 partition" (or however many you want) from the "partition layout" dropdown. Before you apply the changes, click the "Options..." button and choose the GUID Partition Table scheme. Then apply the changes (obviously, this wipes the disk) and restore OSX to the newly created system partition. This will create the GPT, protective MBR and EFI System partition.