First, maybe you should consider a TS3210 model instead which comes with NAS class drives and considerably better performance than the 1000 series and might not be a great deal more after you buy a TS1200 and then buy two additional drives on top of that.
To answer your question, you would need to first delete the RAID array prior to starting. Next, remove one factory drive and insert the new drive. Format the new drive. Once that's done, remove the other factory drive, insert the other new drive and format it. Once that's done, then create a new RAID 1 array with the new drives.
Keep in mind that at this point any drive failures will not be covered by the system warranty.
Just looking at Amazon prices the difference between a TS1200 with 1TB drives and buying 2 4TB WD red drives vs a 3210 with 2 4TB drives is about $100. For that hundred you get a much faster unit, more features, and full warranty support.