You can clone the drives with dd - I've done it a lot and it's the preferred solution. Instead of dd use gnu ddrescue (not dd_rescue) as it's just as fast, gives progress and, should errors occur, it will both retry them and bisect error areas.

If you *do* get errors then you need to know what blocks are not copied and you can tell md to resync just those blocks.

Why are you replacing them? Do you have SMART errors or have you been having problems? I ask as you *may* find that newer disks are less reliable than older ones - I get the impression that consumer grade disks run closer to tolerance nowadays as the margins get thinner. No, I have no reference to stats

Having said that I'd think about getting larger disks and find a way to use raid6.

When a drive fails in a raid5 and you remove it and add a spare. When that happens you then rely on a full read of every bit on the remaining 4 disks - if even 1 block fails you are SOL and relying on fsck