ducky
You could accomplish this with a live boot and gparted! Just be sure to reconfigure grub. I'm heading out the door, but I could share steps tonight (I'd hate to forget a step and leave you in a bad state...).
Alternatively, if you're okay with starting from a new Ubuntu installation, you could run the Ubuntu installer on the drive Windows is installed to. The installer will prompt you, asking if you'd like to dual boot.
I prefer this second approach, but what you described is totally doable, too.