No need to uninstall GRUB to install a new one...
1. Install SUSE without a bootloader and then configure the existing grub to point to the new install. To do this, you edit the file: /boot/grub/menu.lst (In the Ubuntu directory)
2. Install SUSE and let it install the bootloader. In the process, it SHOULD detect the Ubuntu install.
Backup important files before doing any of this.....