You can override Character Movement Component in the constructor.
Instead of use default empty constructor, replace:
AMyCharacter::AMyCharacter()
to
AMyCharacter::AMyCharacter(const FObjectInitializer& ObjectInitializer) : Super(ObjectInitializer.SetDefaultSubobjectClass<UMyMovementComponent>(ACharacter::CharacterMovementComponentName))
Although it is for an older engine version, the Rama wiki it’s a usefull resource:
https://wiki.unrealengine.com/Custom_Character_Movement_Component