I had just done that with XP & Mint7 on one harddrive.
I was just messing around trying to see how bad I could mess up
an old computer I was playing with.
First I booted the live CD and opened the Partition editor.
Then I "moved" the XP partition to the end of the drive and reduced the size
from 100+ GB to 40 GB. (I was sure it would trash XP .....)
Probably should not have "moved" XP partition but I wanted to see what would happen.
Then I did it I did a fresh install of Mint at the begining of the drive with 256 MB Swap and rest for Mint
and the Grub was figured out by Mint for me.
Everything worked fine for me.
Of course I did "defrag" on XP before begining.
The only problem I can forsee is that if Mint is already installed it may
bug out your Grub.
It seems resizing partitions is not as dangerous as I had expected.
The only problem is it took HOURS...............
Things like this are just too easy ....
I demand more Problems