How can I modify the character movement component in UE 4.9?

Cool, worked for me on Unreal 4.13. At first I couldn’t believe you could actually replace a class just like that, but the doc clearly states it uses the static FNames to identify and replace default classes.

Final code:

MyCharacter.h

#include ...

class AMyCharacter : public ACharacter
{
	GENERATED_BODY()

public:
	AMyCharacter(const FObjectInitializer& ObjectInitializer = FObjectInitializer::Get());

    // ...
}

MyCharacter.cpp

#include ...
#include "MyCharacterMovementComponent.h"

AMyCharacter::AMyCharacter(const FObjectInitializer& ObjectInitializer) : Super(ObjectInitializer.SetDefaultSubobjectClass(ACharacter::CharacterMovementComponentName))
{
    // ...
}