Dual-Boot And GPT Partitions

So, currently, we're trying to dual-boot Windows 10 Technical Preview on a Toshiba laptop with Windows 8.1. We already got the partitioning set up and everything, but when trying to install, it says we can't install to a GBT partition, or something along the lines of that.

We went to try to install it the same way that you do for Windows 7 and everything else, but upon setting the disk drive to the boot device, it still loaded into Windows 8. After a little Google work, I found that Secure Boot must be turned off, and that the BIOS option has to be changed from UEFI to CSM. After doing that, we could boot into the Windows 10 disk. However, when trying to install it, it says "Windows cannot be installed on this disk. The selected disk is of the GPT partition style"

We would like to fix it, and I found a way to do so on a Microsoft forum, and that's as follows:

1. Boot up to installation DVD/CD.
2. Click install but don't follow through.
3. Press SHIFT-F10 to bring up console.
4. Type "diskpart"
5. Once inside diskpart type:
-> list disk (find the one you want to convert)
-> select disk 0 (select the one you want from the list)
-> convert mbr (should take a second or two)
-> quit
6. Continue with install

But I need to know if it will format the entire hard drive, as I'd prefer not to lose the data on the primary partition. If we can do it while only formatting the partition for Windows 10, that would be fine (as that's empty already anyway).