The safest approach would be to use the upmap-remapped.py tool developed by Dan at CERN. See [1] for details.
The idea is to leverage the upmap load balancer to progressively migrate the data to the new servers, minimizing performance impact on the cluster and clients. I like to create the OSDs ahead of time on the nodes that I initially place in a root directory called ‘closet’.
I then apply the norebalance flag (ceph osd set norebalance), disable the balancer (ceph balancer off), move the new nodes with already provisioned OSDs to their final location (rack), run ./upmap-remapped.py to bring all PGs back to active+clean state, remove the norebalance flag (ceph osd unset norebalance), re-enable the balancer (ceph balancer on) and watch data moving progressively as the upmap balancer executes its plans.
We do exactly that also, sometimes using pgremapper instead of upmap-remapper.py, but the effect is the same. Make the changes with norebalance, upmap the PGs to be happy where they are until we unset norebalance and let the ceph balancer correct it X% at a time. -- May the most significant bit of your life be positive.