I'd suggest trying to use some sort of crossover server, You can just copy the Appstacks from one server to another but if you "remove" it from VCenter by backupping to windows it will simply get thick provisoned, I don't believe there is anything you can do about it.
The fact that you get the "cannot open for writing" error is because it is delete protected. If you want to you can change it by opening tha .vmdk file (NOT the flat file) and changing the value from ddb.deletable=false to ddb.deletable=true. Do keep in mind to change it back otherwise you could end up with a deleted appstack when you remove a machine where this appstack is still attached.
That being said I would suggest just using vcenter to copy it, saves the hassle.:)
A good way to do this is create a NFS share and mount it between hosts in both prod and POC vCenters. Rescan the storage in both AV prod and POC and mark the new NFS datastore as non-attachable (this will prevent Appstacks in NFS from attaching).
Create a storage group in each environment which contains the local AppStacks datastore + NFS; you can set to automatically import/replicate or manual.
AppStacks will be replicated between POC > NFS, and then NFS > prod. This is the same method used when deploying to multi-site environments.
You may be doing this already...
When you copy them over via WinSCP each app will have 3 files (.vmdk, .vmdk.metadata,-flat.vmdk)
In WinSCP right-click the .vmdk and select 'File Names' -> 'Copy to Clipboard (Including Paths)'
Then when you SSH into the ESX host to thin provision just do 'vmkfstools -K <"paste in the whole path you copied from WinSCP">
I would also triple check that the AppStacks aren't attached to any machines when you try to export them via WinSCP.
Better do this with powercli.
If you do this on a vSAN datastore your copy will be corrupted
Created a script back in the days to do this. Not the best coding i have done but worked great.
I need to test this again since I haven't test this 5.5, but I'm pretty sure the core command(copy-datastoreitem) in this script, doesn't work between vcenter environments. II have a similar one that I use, but it requires downloading the vmdk from one vcenter and uploading it to the other one, which loses thin provisioning.
yeah, I had a shared NFS lun on both sides available. Made life easy to transport the appstacks between environments.