Ive read some posts here on how to modify multiple files in a directory using a for/while loop with grep..

How would one go about doing a search/replace for a portion of a directory name..

ie..

"Change all instances of images to new_images in directory "x" and all subdirectories"

/home/directory/images/thumbnails

to

/home/directory/new_images/thumbnails..

Is this possible without writing a script?

Why don't you just do
mv /home/directory/images /home/directory/new_images

The problem is I have to "find" these directories in a nest of several hundred directories.. So it would take hours to manually go through the directories recursively and find/rename them.. I was hoping a script command could do it for me

Stupid script to try...
WARNING if you have images/images then you may run into trouble. or /home/foo/imagesthatIdontwanttochange/images may be changed to something you don't want because it contains images...

echo "this will generate a script to rename images to new_images"
find . -name images > /tmp/junk123
find . -name images | sed 's/images/new_images/' > /tmp/junk124
paste /tmp/junk123 /tmp/junk124 | sed 's/^/mv /'
rm /tmp/junk123 /tmp/junk124

This script will just spit out the output to the screen. If you want to redirect it to another file then execute the file that's up to you. If you want to highlight the output and do it that way it's up to you... I would certainly test and retest to make sure it is doing what you want. before you do it on a production environment... hope that was enough of a disclaimer...

edit: I am also assuming you have a base directory you will start from and not just searching from /