Both are good options, a new data center in same cluster will work faster as you can stream multiple nodes.
Or you can add 8 nodes to existing data center and then decommission one node at a time but this is lengthy process including cleanup after decommission.
One node at a time is equally good. If data isn’t much this step can be efficient.

One approach we did was:
RSync a temporary storage volume online
Stop c*
Rsync delta
Detach temp storage
Start a new node with new hardware
Attach volume
Rsync to actual storage

Repeat process on each node one at a time
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