I’m sure you’ve been in the situation (I have many times), where you install an os, use it for a while then move on to something better installing it to a new partition on the hard disk. After a while of trying it out you decide you wish to move to the new linux distro completely. But you still have the old grub boot set up, which will boot by default into the original distro. This is easily changed, but even then, its much better to have your main distro controlling grub allowing it to update when new kernel updates are made or if you upgrade your distro version.
This is a really easy guide for how to do it that takes you through it nice and gently http://community.linuxmint.