Do you mean your /var is growing bigger and you want to mount it on another larger partition? If yes, you just need to change /etc/fstab.
For example, you currently have,
mount /dev/sda1 /var
and /dev/sda1 doesn't have enough space. So you just change like this, (if /dev/sdb1 is a larger partition)
mount /dev/sda1 /var
mount /dev/sdb1 /mnt
cp /var /mnt -r
then change /etc/fstab to,
/dev/sdb1 /var
then reboot..