you can replace debian with ubuntu, with a little of work... is not easy, nor friendly, but it's possible.
first list all debian packages (
dpkg -l >/root/debian.pkg.lst)
You can edit the
/etc/apt/sources.list and replace the debian with the ubuntu sources.
apt-get upgrade ;
apt-get dist-upgrade you should have a near ubuntu.
Try to manually fix problems with aptitude and try removing debian packages (check the above debian list file to compare). then try to remove all the debian packages (again from the above list). If possible, try to postpone the reboot to later, until everything is fixed
The closer related are the debian and the ubuntu, the better.
Another way is to partition the HD to have some free space. Then configure a virtual machine that uses the real HD as a virtual HD. Taking care to not touch the running partition, boot the VM and connect to the screen on another PC. Boot the ubuntu iso and start installing on the free partition. don't touch the current debian partition, but install the grub as normally.
After everything installed, reboot the VM to confirm that the ubuntu can boot and is the default.
If everything is ok, reboot the real machine and it should boot from the installed ubuntu. After booting correctly, you can remove the old debian partition and expand the ubuntu filesystem and any fine tune you may need.