Yeah, personally I would keep the 1st gen 8 with the 5 x 6TB drives as is, create a new system using the 2nd gen8 and purchase all 8TB drives. When ready, copy the data from the 1st system to the 2nd system and then you've got both, redundancy intact and at the fastest possible expansion time.
Of course, if you're not planning on purchasing ALL 8TB in one go, or your space will be used up before then you can just upgrade system 1 with 8TB drives until its maxed and then add the spare drives to the second system... guess it comes down to how much can you outlay in HDDs at one go & how quickly you will use the space up that remains on the first system.