This sounds like the most logical way to do this. (Providing it works.)

OR

Can I take 2TB drives out of the original TwinStore, copy them via my computer to the 3TB drives, Then put the 3TB drives into the TwinStore?

Control:15, TwinStores (2) w/Seagate ST33000651AS 3TB Drives, SourceOne, Drobo D5 Storage,
Messenger Preamp, Ayon Skylla II DAC, Gamut D200 MKIII Amps,
Dali Euphonia MS-5, CS-4, AS-2, RS-3 Speakers.