Jayesh,
Check out the DTA2800. It is a pretty slick box.
The documentation can be found here:
https://support.netapp.com/documentation/docweb/index.html?productID=61318
This provides migration assistance without having to do a host based data moves.
Have a great weekend!
-Rich
EDIT: Just so you know this does exactly what you are looking for. A data migration while the hosts are online and the migration happens in the background. The last I heard is that all you needed to do was a reboot when everything was synced up and the host would attach to the new LUN's. This might have changed to be more seamless but I am not 100%. Also, last I checked, you didn't have to buy the device, a NetApp certified DTA guy could come to you, set it up and help you do the migration then leave once everything was done and take the device with them.