Setting up AG Groups with Current Failover Cluster
For your first question, yes you can use the same environment (Windows: 2012 R2 Standard and SQL 2014 Enterprise Edition) but the configuration should be changed. During failover clustering, primary and secondary servers shares the same resources or drives, this didn’t work for Always on configuration, and both primary and secondary replicas has to have their own drives. 

For your second question, it depends on your preference and available resources, if you think that it’s easy just to reset drives on both servers and change configuration you could do that. If you have sufficient resources, space in SAN and feels more confident to start a fresh install you might chose the second option. 

Concerning about the licensing. Assuming you are planning to use secondary node only for disaster recovery or short maintenance time, you need licensing only for a primary node. However, if you are planning to query from secondary node, you are also required a license for the secondary node too.