Do you mean a TS-664? 
First, if I were you, I would spend an extra $100 and buy a TS-673A. You can go up to 64 Gig of memory, the CPU is a little bit faster, 2 PCI slots, it can run QuTS Hero, etc. But only you can decide on what you can afford.
Second, I don’t think it’s possible to upgrade a RAID1 to a RAID5. It’s a completely different sort of storage architecture. So you will really need to back up your data, wipe your drives, create a new pool that is RAID 5 and off you go.
And since we are probably a few months from QTS 6 being out, yeah, I would update.