If you are using Shared datastore, you don't need downtime for storage vMotion. Perform below steps to perform hot migration.
- Right click on VM
- Click on Migrate.
- Migrate Storage
- Select Second Datastore where it has to be migrated.
If storage is not shared, need downtime.
- Shutdown VM.
- Right Click on VM
- Migrate
- Select Datastore and Host Migration.
If you have already performed above steps, and still showing old datastore, probably your vmx, swap, or some of the files does exists in old datastores.