1. A method for issuing a command to deallocate free storage blocks previously allocated to a logical block device for a hypervisor of a virtualized computer system in connection with an upgrade of the hypervisor from a first version that does not support the command to a second version that supports the command, comprising: upgrading the hypervisor from the first version to the second version; after the upgrading, identifying a set of free blocks to be released, from a data structure maintained by the hypervisor that indicates blocks that are free and blocks that are used; allocating the free blocks in the set to a temporary file and marking the free blocks allocated to the temporary file as used blocks in the data structure; and issuing a command to the logical block device to deallocate the free blocks allocated to the temporary file; wherein the hypervisor supports execution of one or more virtual machines, and is upgraded from a version that does not support thin provisioning to a version that supports thin provisioning.