Viewing my datastores from the EMC Unisphere console, on the Datastore Capacity tab.  I see where you can place a checkmark to enable virtual thin provisioning. 

Thin Enabled

Thin Disabled

1. What exactly will this do to an existing datastore

2. Can I do this in a live environment?

3. Should I vMotion the vm's to another datastore first?

What I want to achieve:  I have two storage pools, with exactly the same disc in both.   I want to to delete pool 2 and then reprovision those disc into the remaining pool so that there is one large pool.