[michaelt]

  haven't people written tools to help with this?

Let's say you have a web page with a javascript slippy map that imports openlayers from a CDN; and openlayers then retrieves map tiles from openstreetmap.

If you serve that page over https but the javascript CDN url is http, the javascript library won't load. And if the js CDN supports https and you switch to it, the library might still compose a http URL to retrieve the map tiles - causing some browsers to block the tiles as mixed content. Other browsers are willing to load http images on https pages and will work. Unless the tool understands how the map library composes its URLs, someone will have to fix this manually.

To detect bugs like that automatically, after changing to https you'd have to spider every page in your site with several different browsers / browser configurations looking for errors and bad links. And if your archived site had a bunch of errors and bad links to start with, you'll need some way to compare the before-and-after error reports too.

TLDR: It can be more complicated than you think.