Since you have laptop, replacing HDD wont be an option. Installing Ubuntu over windows works well as ubuntu bootloader detects the windows entries easily. Vice versa will probably leave you with no option to boot into ubuntu unless you reinstall GRUB https://help.ubuntu.com/community/WindowsDualBoot
Preferably install on a separate HDD other than the one where Ubuntu is installed. Disconnect ubuntu hdd when you install. Otherwise you run the chances of overwriting ubuntu bootloader.
Once you have installed on separate HDD. Just set either of the HDDs as first boot device and manually update the new os entry (in grub if you choose ubuntu hdd as the first boot device, otherwise if you select windows hdd as first boot device then use bcdedit to add new os details). Adding entries require certain level of expertise. If you are not sure then let us know.