Alternative code optimization

[mnp-mid]

Extracted switch case code into multiple methods

[MichaelKaaden]

That isn't an optimization. Effectively, it hides what a switch makes clearly visible.

[mnp-mid]

The benefit of having methods instead of a switch case is that you are able to use inheritance at any time and overwrite a single method. This may bring some flexibility in future, if required.

Additionally, a method is encapsuled in it's own and not interfering with other case definitions. So variables with the same name doesn't need to be defined out side the switch case.

Besides it would fit into the open/closed principle. Open for extensions, but closed for modifications.

[MichaelKaaden]

Simply no. You don't override anything in a reducer. Using multiple implementations here would be completely wrong. I see your point regarding variables, but if you really need to do so much logic, it probably would have been better put inside the ActionCreator.
Remember: A reducer is a pure function, not a complicated class construct.