Thanks Allot.
From that article they state the following:
"...Upgraded VMFS-5 partitions will retain the partition characteristics of the original VMFS-3 datastore, including file block-size, sub-block size of 64K, etc. To take full advantage of all the benefits of VMFS-5, migrate the virtual machines to another datastore(s), delete the existing datastore, and re-create it using VMFS-5."
This basically means, go the host configuration, storage, right click datastore form the list and select delete. Then go add storage and add the same LUN back only formatting it with VMFS 5 this time around?
And all this without unmapping and creating a new LUN on the SAN?