Say if initially the replication factor was 2 and later I change it to 3. Will the old data also be replicated with replication factor as 3 Or only the new data will have the replication copies as per replication factor as 3.
The cluster master will always ensure that the cluster is in healthy state given the current configuration. So yes, in a single-site cluster, increasing the RF/SF will cause the cluster master to initiate fixup activities. Depending on how much data you have retained in your cluster, you will see an impact to your cluster's search and indexing performance.