Change drive letters

I have a dual boot system for Vista & Win 7. Each OS has their own hard disk drive. I unplugged the Vista drive while installing Win 7.
In Vista the drive letters for Win 7 HD are J:, K:, M:, and N: when I boot into Win 7
the drive letters are F:, D:, C:, and E:. (M: becomes C:)

I tried to change the letters to J, K, & N with partition tool but those letters are not made available. Do I need unplug the Vista drive and change the letters with only Win 7 running.

I want to use files in these partitions from both Vista and Win 7 for the time being but if the letters are different I run into a problem. Any ideas.

Thanks

I tried changing drive letters but J, K, and N are not on the list of letters I can use.

Sorry, my own stupidity. Letters J, K, N, were assigned to DVD drives and a USB drive. These drives don't show up in partition software. All is well. Thank you all. Now I can re-assign my user files.