Unable to reinstall to previous system partition (GPT)

I tried to reinstall Windows to my existing system partition (I would like it to make one of those Windows.old folders). When I select the system partition, I get the error:

Windows Setup

Windows cannot be installed on this disk. The selected disk is of the GPT partition style.

This is a bit stupid since Windows made this partition during the original Windows installation. My motherboard has UEFI.

When I press Shift + F10 to being up the console and use diskpart to try to convert the disk to MBR, I get:

Virtual Disk Service error:
The specified disk is not convertible. CDROMs and DVDs are examples of disks that are not convertible.

The disk in question is an SSD.

I really would like to proceed without deleting the existing partition because then I'd have to find a way to copy the contents of the disk and enough space to copy the contents to, just in case I later need something I left in a program folder or whatever.

Thanks for your response. I tried installing whilst running Windows and the installation failed with a vague error message. I then tried the 'Refresh' function after which my PC boots to a black screen so attempting to install from within my old Windows installation is long longer an option.

I don't think so. I was presented with two options for the same disk and one had UEFI written in it. I thought it was a bug. It certainly isn't explicit what adding 'UEFI' to the partition name means when selecting what partition to boot from. Looked like a bug to me. Thanks. I went back and selected the UEFI alternative and it seems to be working in the expected manner (and said it would create a Windows.old folder).