You can set how much gets cached so the OST doesn't fill up. We have ours set to 3 months, and haven't had any WV's fill up using the standard UIA only WV. If you need something older than that, it will download it.
However, if that's not suitable, I'm sure you could take the writable template files uploaded to your LUN and expand it:
vmkfstools -X 25g /vmfs/volumes/xxxx/vmname/vmname.vmdk
You will probably need to attach it to a vm to expand the partition, but make sure you don't upload any new templates when upgrading and just zip up the contents of the new WV template and import it under the WV section of App Volumes manager.
Here's a kb for expanding the vmdk:
VMware Knowledge Base
And for updating the WV:
Update Writable Volumes
