You can probably go through these steps to get them to open. The reality is that so much has changed especially since RW 5 that you probably would be better off just copying and pasting from the existing sites,
The important thing in upgrading is to get the addons (stacks, etc) current and copied across to the new RW8.
The old sites are probably so dated and at least the RW5 ones aren’t responsive. I’d probably start over with a new theme or framework.
https://help.realmacsoftware.com/en/articles/2201707-upgrading-from-rapidweaver-7-or-earlier
https://help.realmacsoftware.com/en/articles/1905164-upgrading-from-rapidweaver-5-or-6
https://help.realmacsoftware.com/en/articles/1905613-missing-resources-when-upgrading
https://help.realmacsoftware.com/en/articles/2093782-how-to-manually-install-add-ons
https://help.realmacsoftware.com/en/articles/1904820-purchasing-an-upgrade