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:
And for updating the WV:
Thanks for the suggestion. I already created a new template with the size I want and uploaded it to the cloudvolumes\writable_profile location of the datastore. it works but the system just doesn't automatically use it for new volumes. I even renamed the original and saved my new template with the default template_uia_plus_profile.vmdk name but still no luck.