As already stated, you can use the existing GRUB install to load any of your Linux versions.
You can install GRUB such that it points to any of your Linux partitions to find its files. Here is the basic procedure:
grub #opens the GRUB shell
root (hdX,Y) #tells GRUB where "home base" is going to be--ie where it will find the /boot/grub directory with the config file, stage files, etc.
Note that GRUB numbering starts from 0.....If you want "home base" to be hdb2 and GRUB stage1 installed on the mbr of hda, the above commands become:
See my "booting" link below (includes a link to the GRUB manual)