Basics

1. Create a copy of your site, and only work on that

Prepare Templates

1. Move all templates in the main folder into a subfolder "old"
2. Adjust all view-configurations to point to the new location
3. test the website to verify everything still works

Prepare Content-Types you want to Drop

1. Add OLD to the front of the shown name so it's easy to spot these in the list

Prepare Content-Types you want to Upgrade

Do this only for the content-types which have a lot of data and which you want to keep in this schema. Don't do this for content-types whose data you'll manually copy paste anyhow.

1. Verify the static name is the same as the static name of the upgrade you'll add later on (this may be very technical, make sure you know what you're doing)
   1. If the static name does not correspond, rename the old content type on the existing installation
   2. After changing the content type static name, re-assign the content type for all templates
2. Review the field names of the old / new schema, and rename the old field names to match the new fields for import
   1. This will probably require that you also adjust the old templates to use the changed field names
3. Verify everything still works