To preserve your data as long as posible, Bacula will just "erase" the volume file content (and change the volume file size in filesystem and catalog) when it is reused for a backup. You will just notice the truncate action when you execute a job that will use this volume for writing.
After changing the recycle flag to yes and reloading the new configurations for the pool, you will also need to apply the changes to all the existing volumes: update volume -> All Volumes from Pool or All Volumes from all Pools.
Best regards,
Ana