I have a new Dell XPS 8930 with UEFI boot C: drive and an MBR C: drive from my old Dell XPS. They're both Win 10 Pro but the older is 1709. I'd like to convert my older C: drive to GPT and have the ability to boot from it also, if Pro could do this.

Hello Bruce, Thank you for reaching out. We would need the computer in-shop for an assessment but we offer a free diagnosis and estimate within 1 to 2 business days after it has been checked in. The technician thinks he can convert the older drive to GPT but you'll only be able to boot off of one drive. Jake Sain - Client Services Coordinator

I think you should call to ask them.