@Wimster Yes, but it takes much longer and requires a very good knowledge of your future distro.

You need to manually unpack packages of your future distro ijnto soem folder to create a chroot environment that is able to run the package manager of your future distro. Chroot into that and install the software for a minimal bootable environment. Move your new distro environment to the root dir of your partition, remove the old stuff and configure your boot manager. Then you can boot into your new minimal distro and install whatever you want.

Debian has some programs to help you with the first steps, like debootstrap (for .deb based distros) and rinse (for .rpm based distros, like OpenSUse).