This in not tested, but should work...
1. Remove one of the drives from the RAID, and place it
and one of the new drives in a different computer.
2. Use
dd to copy the old drive over to the new drive.
Code:
dd if=/dev/olddrive of=/dev/newdrive
The new drive is now a duplicate of the old drive, including
size of partitions, so you need to resize the partition(s).
3. Use a tool like gParted to resize the partitions to your liking.
4. Remove the other old drive from the RAID, and replace it with
the new copyed drive and the second new drive.
5. Now you can rebuild the RAID as you would if you just replaced one drive.
HTH