When I moved my blog to a domain I owned I added little notes to my old content saying "Previously hosted on ..." in the hope that searches for that content by URL would find the new homes.

Example: https://simonwillison.net/2004/Jan/22/defendingWebApplicatio... - at the bottom it says "Previously hosted at http://simon.incutio.com/archive/2004/01/22/defendingWebAppl..."

I just tested it and it works! https://www.google.com/search?client=firefox-b-1-d&q=http%3A...