NAS542 - Replacing all disks

MarVel

I recently bought a slightly used NAS542 with 4 x 1TB disks in RAID5. 
(currently ~85% full, latest FW installed)

I'm planning to replace all 4 disks with 4 x 3TB disks but I'm a bit unsure how this is done smartest way without losing data.
Just pull one disk at time, replace, add to raid-set and wait a day until resynced and repeat 3 more times?
 Or is there a better/smarter way to do this?

All advices welcome!

#NAS_Aug

Wiasouda

From Zyxel user manual on page 44. ftp://ftp2.zyxel.com/NAS540/user_guide/NAS540_V5.21 ed2.pdf, you have to replace it one by one after resync done.