Have a new (used) My Book Duo coming... QUESTION about drives

HI folks

I have had a My Book Duo for 9 years (WDH2U10000N) and it is GREAT, but SLOW thanks to being older USB tech. I have a used USB 3.0 version on the way and want to know if I can just swap in the 2 drives and that’s it. OR will the device want to format them. I can’t loose what is on those drives, and I do have some backups, but I am praying all I have to do is swap them in and be in business.

Thoughts?

For the record, when I installed my 2 drives into the updated enclosure, I had to do a complete reformat and “re-install” all the files back onto the drives once they were in their new enclosure. I don’t quite understand WHY, but that’s how it ended up happening. Can anyone explain in plain English why I couldn’t just install the drives and use them?