Business Manager Package Upgrade/Downgrade

[geeza@]

Hi Guys, When using the upgrade option (upgrading an existing bronze hosting package to silver) in the Business Manager it fails to upgrade. When I go into the 'MBAPI Command Output' for the errored upgrade event I get the following:

<errors>
<error>
<title>Panel Error</title>
<message>ProcessPanelCommand: Plesk Client Upgraded Error: "pleskunix"</message>
</error>
</errors>




<queueMessage>This is not a relevant call (panelUpgrade), let it go.</queueMessage>

The package I'm trying to upgrade is on an external Plesk 8.6 server and the packages are identical except for a couple of increased values (like the number of domains allowed) on the silver package. Both packages provision fine on their own (and are using manual values not templates), it's just upgrading and downgrading them which isn't working. Does anyone know what is causing this or having the same problem? Thanks