I suppose that you could do that but I think that there is an easier way, depending on how much configuration you've done with Ubuntu. I am guessing that you want to eliminate the Mandriva partition and merge the freed space into an existing partition.
On the Ubuntu system edit the /boot/grub/menu.lst file to remove the entry for Mandriva. Next I would perform a full system backup. The backup should include the master boot record for the disk. You could use partimage for this. Partimage saves the disk master boot record automatically as long as you don't select bz2 compression. You will have to boot a live CD to use partimage. It will not back up a partition that is in use. The backup is going to be required regardless of how your disk partitions are laid out.
The next step depends on how your partitions are laid out. If the Mandriva partition is adjacent to the partition that you want to expand then you can use parted or gparted to do this. I don't want to write the exact steps because instructions are available on line.
If the Mandriva partition is not adjacent to the partition that you want to expand then you will have to edit the partition table with cfdisk to move things around. Then you will have to format the new partition and restore the contents from your backup.
The method that you asked about, reinstalling the whole system, makes sense if you didn't do much system configuration. Otherwise performing a backup and doing what I recommended might require less thinking to reconfigure a fresh system.
