The normal way to move a volume from one mount point to another is something like:
1) Create the new mount point:
2) Unmount the SMB share from the old mount point:
3) Open /etc/fstab in your favorite text editor and change "/mnt/nas" to "/nas":
4) Mount the SMB share at the new mount point:
5) Remove the old mount point since you don't need it anymore:
But that probably won't solve your real problem since "/nas" is still under "/".
What program are you going to use when you're making the backup?
Some of them have options to either skip a directory (in your case you would want to skip "/mnt/nas" and/or "/nas") or to stay on one filesystem (in your case you would then have to make two backups, one of "/" and one of "/boot").
If possible, I would suggest finding a backup program that lets you stay on one filesystem. You most likely also want to skip "/dev", "/proc", "/run", "/sys" and maybe more. That's a lot of typing and you're likely to forget to skip a (pseudo) filesystem.
It's much easier to specify that you want to backup this filesystem and stay on that filesystem, even if it means you'll have to run the backup program first for "/" and then for "/boot".
For "cp", "rsync" and "tar", the option you're looking for is "--one-file-system".