Thanks a lot for your advice @DMehaffy
Updating the code and getting rid of whatever old stuff from beta releases was very simple. At the same time I switched from mongodb to postgres.
I created a new strapi project, selecting postgres as the database. Tested it and made sure it worked fine.
Then copied the ./api and the ./components from my previous strapi into the new.
I needed to rebuild strapi using the cli and it worked fine 
I’m using graphql to access strapi data and I found that all my code worked just fine. The only change I have found so far is that in mongodb the date field was named updatedAt and in postgres the same field is named created_at.