Drobo 5D Replacing 2TB's with all 8TB - Fastest Way?

I have been very happy for the past 3 years with my 5D (and five Seagate 2TB drives. As I got to 80% used (4TB/5.27TB) I just got 5 Seagate IronWolf 8TB NAS drives. I pulled my bottom 2TB drive out and replaced it with an 8TB and it says it will take 8 hours to finish. I will need to do that with the 4 remaining drives, one by one. Is there a faster, method? I guess I could copy everything onto one of the 8TB drives on a USB and install only 4 of the 8TB drives into the Drobo and then copy everything over? ANy other suggestions how to do this? thanks in advance

I think you answered yourself correctly. Copy off then copy back.