Warning: RUN ON SENTENCE

It seems like if you can boot from an alternative startup disk, which could be a netboot or a different disk of any type (thumb, ssd, 2ndary HD, cd, dvd... whatever; which has a disk utility... then, you could format the drive with the partitions at the top level (not down in the individual partitions);

which would then enable you to install your new os to it.

If you can boot from an alternative startup disk then, you could format the drive with the partitions at the top level (not down in the individual partitions).

Which would then enable you to install your new operating system to it.

The alternative startup disk could be a netboot or a different disk of any type (thumb, ssd, 2ndary HD, cd, dvd… whatever; which has a disk utility…)