You have a couple of options, you can easily add two more 1TB drives and expand the current RAID 5. If you want to replace the existing array with larger drives, you will need to replace each disk one at a time waiting for the array to recover between each disk swap. Once the drives are all replaced and the array is healthy you can then expand the array to include the additional space. Then you have to modify the OS to utilize the new space..
What you have to consider is what is faster, the above scenario, or rebuilding from scratch and restoring the data from backup.
Regardless of methd make sure you have a verified backup before you start making changes to the array