Hi - I'm not sure what the definition of "inactive" is for the purpose of moving landing pages to a new subdomain, since I have never done that before. But have you tried changing the landing page visibility?
If you use the two arrows button (next to Save) and then select 'Settings' from the drop-down menu, there is a section for Landing Page Visibility. If you switch it to Internal maybe you can try again to make your change. If that doesn't work, hopefully someone else can assist you soon!
If that Landing Page is called in any of the Campaigns for Reporting, it will show the status as Active. Inorder to change the status from Active to Draft, deactivate those campaigns.