I guess you'll be getting a backup solution in the very near future.
1 - back up the data
2 - remove the existing 1TB pair
3 - install the new 2TB drives and format
4 - restore the data.
If you attempt to swap the drives one by one will result in a 1TB RAID1 volume and a 2TB JBOD volume - if you're feeling lucky, you can remove one of the existing drives and connect it to a PC running some form of Linux, or Windows with a ext2/3 file system driver and try to access the data that way - if successful, it would remove the need to find storage space and perform step 1.