I recently migrated a few Web Apps from one subscription to another. In the manage.windowsazure.com portal everything works as expected, however in the new portal.azure.com preview the migrated items still show as belonging to the old subscription and don't function when selected.
Also one of the migrated Web Apps has two Application Insights items attributed to it, one for the old subscription and another for the new subscription. One has a resource group of "Default-Web-WestUS" and the other is slightly more lower case "Default-Web-westus".
Figured I mention it in case it's of interest to the engineers for investigating.
Apart from that, I'm loving the new portal for the items it functions for.