What is the best way to switch namespace from in-memory mode to SSD mode given that we have a live system? Is it possible that the same namespace is configured to operate in-memory on the 1 node and on SSD on another one?
Based on what you asked, I take it you have two nodes? Hopefully you have replication factor of 2 on (default), and to switch namespace from in-memory mode to SSD. You will have to bring down one of the node and wait for migration to complete. After migration has completed you will want to change the in-memory to SSD. After that bring up the node and you should be fine.
Also you can configured Aerospike to operate in-memory on one node and SSD on the other, which is fairly common from what we see.
Hope this helps, if you need anything else let us know.