PayPal has errors when trying to upgrade membership levels:
"We are unable to modify your subscription. This may be because your existing subscription calls for no more payments to be made or because it has been"
To fix it change the two occurrences of
<input type="hidden" name="modify" value="2"> to
<input type="hidden" name="modify" value="1"> on gateway.paypalexpress.php - see https://premium.wpmudev.org/forums/topic/paypal-error#post-156675 for more info.
Please pass this info on to your developers so that they can fix it in the next upgrade.