You will need to write out all the fields, the record update syntax no longer allows you to change the type of the record you’re updating by adding additional fields or changing the type of fields.
This was something that existed in an older version of Elm, but it caused a lot of confusion, made compiler error messages harder to do well and was a very rarely used feature of records.
People from an OOP background often see extensible records as a way to start modeling the inheritance style they’re used to but they’re really not designed for that.
Richard has a really nice talk about using extensible record types that will help you get a better idea about what they’re designed to provide.