Why 3.3.9 which is a bit old. https://neo4j.com/release-notes/neo4j-3-3-9/ reports it was made available November 2, 2018. Upgrading from 2.3.12 is great since its even older but by going to 3.3.9 you are upgrading to a release that is 1.5 yrs old.
As to the upgrade I would suggest a 2 step process as follows
a. first upgrade to 3.3.9 ( or a newer release ) and remain on HA Clustering.
b. once you are satisfied the upgrade to 3.3.9 ( or a new release ) on HA Clustering is satisfactory then convert to Causal Clustering. The conversion from HA to Causal should only require changes to your conf/neo4j.conf. The contents of graph.db remain the same whether in HA or Causal.
Architecturally it should also be noted that HA is different than Causal in that HA has a Master and Slaves whereas Causal has Leader and Followers. Further under HA a write can be sent to any cluster member (though ideally it is sent to Master) but Causal can only accept writes at the Leader. Causal along with the Neo4j Bolt Drivers does allow for a connection with a url of bolt+routing://<Neo4jInstance>:7687 and when including +routing in the url rather than simply bolt://<Neo4jInstance>:7687 then when a write transaction is submitted it will automatically be routed to the cluster member which is the leader even if the leader is not at . For this to occur however the Cypher needs to be sent explicitly as a write transaction, i.e. session.writeTransaction().
