It is not as simple as "turning on EFI" -- an EFI system requires an
EFI system partition and the boot loaders/managers need to be located there so you will need to create that partition and copy over the files (and mount /boot/efi to the ESP for your Xubuntu system before you update GRUB-EFI).
You will have to re-install Windows.
It would prebably be simpler to re-install Xubuntu as well (after you have re-installed Windows).