Will turboactive provide any facility to crosscheck or cancel previous product key and dispatch new product key?
Yes, you can search for the product key in your LimeLM dashboard, click "Deactivate" on the previous activation, and then the user can immediately use the key on their new computer.
If it provides facility to cancel previous product key and dispatch new product key then how it will work? Is it manual and we need to email the key to user A or it is automatic?
You don't have to send a new product key. The user can use the same key. Our LimeLM / TurboActivate backend handles all the details. You can read more about hardware-locked licensing (and specifically how LimeLM works) here.