I am kinda new at the VM Ware thing and am looking for some help...

We are running ESXi 4.0 on a Dell Blade. Originally, there was a 250 GB LUN created to house the VM's on our SAN. Recently, we added additional storage to the SAN and want to add some VM's to the blade. I have expanded the LUN by 200 GB making it 450 GB, but ESXi doesnt see the new space. I think it is because I didn't expand the file system out to the full size, but left it as it was since I didn't want to risk loosing data. If I expand the capacity to the full 450 GB, will ESXi be able to adjust on the fly with out loosing any data? We have several Virtual Servers running and a small file store that we cant loose.