Join Date:Joined: Mar 2011
There's no simple way. NFC Tags are by nature passive - when your phone's battery's magnetic field comes into contact with the tag, the tag is triggered by a quick burst of power. It happens only once - if you remove the magnetic field, then place it back again, it triggers again. There have been a few custom ROMs that allowed what you're looking for by constantly/actively checking for the NFC tag.
Otherwise, I don't know of anyway to do it. In theory, an app should be able to check periodically (like every minute or two) to see if the nfc tag is still in contact with the phone and once it's not, trigger reverting back. But I don't know of any app that has built that in yet. Plus it might have battery drain implications.