Unmounting/detaching a datastore will leave the data intact. However, deleting a datastore will wipe the VMFS partition!
There are actually two ways to increase the size of a datastore. The first one - adding an additional LUN as an extent - is how it was usually done in the past, where the maximum LUN size was restricted to 2TB. With vSphere 5 the max. LUN size can be up to 64TB, so the way to increse the size of a datastore would be to delete the existing datastore, delete the LUN on the storage system to free up disk space and then increase the size of the existing LUN/datastore. In both cases, the data on the "old" datastore cannot be preserved.