Hi,
Your best option, even if it is going to mean changes for your users one time, is to implement DFS. This way you can have any number of underlying shares that can be moved around even to other machines and just require an update to the DFS information. Such moves will be much easier in the future and you will be able to balance growth and backup windows and such with very little disturbance to your users.
You might be able to add a symlink in the CIFS filesystem itself pointing to the new directory's destination, but I am unfortunately not too versed on such matters.
I didn't have any luck using widelinks a few years back because one needed to, iirc, have the data in a unix qtree, but I might have forgotten the details.
Good luck.