From my limited experience with this I will try to provide advice. As long as you didn't reformat you're drive to a different file system, changing the partition entry in the partition table shouldn't be a problem. The real issue here is you changed from MBR to GUID, meaning it probably erased some important data, since they do not use the same exact areas of the the harddrive to store partition data. So the real answer is without some serious work on you're part, it might be very difficult. When you repartition your harddrive you don't really actually erase anything, you just change the partition table, which in this case can be destructive.
Also if you did not click apply in Disk Utility, it will have done no conversion any way, so you should be alright.