Are you using vCenter and vSphere 5.1 or newer? Do you have a vMotion (not Storage vMotion) license?
If yes to all, then you can simply use Cross-Host vMotion (XvMotion) in the vSphere Web Client (not available in the old vSphere Client C#).
This will enable you to migrate it from one datastore to another datastore while the VM is online (you have to change host at the same time though, but you can always run a normal vMotion afterwards to change back to previous host).
Principal Technologist, Global Education Services @ Veeam Software
Veeam Certified Architect #1 | Veeam Certified Trainer #4 [v7,v8,v9] | Veeam Certified Trainer Mentor #1