Next week we're turning off our old intranet site so I have setup a "Migration Switch" and a redirection page so that from the moment we turn off the site people will not be able to access it. Because if we actually turn off the site we would receive phonecalls with people needing access to their lost data, and because certain parts of the old intranet are reused in our new one, we instead check if the page is embedded, and if it's not check that the user is on a whitelist of specifically allowed users (us). If this is not the case we redirect them.
Next week we will remove all navigation links to the old intranet, turn on the switch, and remove the documents from the search indexer on the (new) intranet. There must be better terminology for old and new intranet.
Hopefully this will ensure that everything works smoothly.