Ok...so, the only way to achieve what you're wanting to do is to 1. create separate disk space (LUNs) on your SAN and add them as datastores then create VMDKs from the disk space and copy the data over to the new volumes (or use your b/u solution to restore the data); or, create the LUNs and direct-connect them via RDM to your VM, but you would then still need to copy the data from the old volumes to the new ones; or lastly, do a conversion as mentioned above allocating only the space you need for your disks.
As "arun" mentions...b/u your data and VMs prior to making any changes, although the steps shouldn't (again...shouldn't) corrupt/remove/delete anything.
Hope this helps.
~coolsport00