Are you talking about breaking the RAID and rebuilding with a new, larger disk? If so, what'll happen is the RAID will only grab the appropriate size of the new disk. For instance, if you had a RAID 5 with 500GB disks and replaced one with a 1TB disk. The RAID rebuild would only grab a 500GB chunk of the new disk.
An issue with breaking and rebuilding is that you have the chance of a second disk failing in your array during the rebuild resulting in a two drive failure causing data loss.
The backup and restore provides the benefit of having a backup if you experience any issues during your data migration.
However, a tested backup strategy should be an integral part of a healthy data security system. See: