ReadyNAS Device Compatibility List

I have my wife using a NV+ as her storage for her photography business. I upgraded it's capability a couple of years ago and went from 500 GBs to 1.5 TBs drives. However, she is getting close to out of space again. I was considering getting her a Pro 6, but wondered if I can move her 1.5 drives straight over and add 2 more, or am I going to have to find an something to store all the data on first, format, and then move?

Thanks for the help...