MBR = master boot record
Every drive has one, and linux partitions can have boot blocks as well.
As long as your bootloader is installed in the MBR of hard drive a (represented by /dev/hda), changing what's on the partitions might break the bootloader's links and make it unable to boot your new debian system right away, but all you have to do is remove the RH boot information from your config file, and replace it with Debian. You won't leave your system unbootable by installing debian, if your loader is on the MBR.
Read man lilo.conf, or equivalent for grub. It'll all become clear.