Only existing keyframes will be adjusted, no new keys will be ever be created, this is to allow tweaking of keys without fear of ever creating new ones. In previous versions of LightWave this mode was available when Auto Key Create Default in Options was set to Off, but the Auto Key button in Layout was active. Now you just need to select this option.
Auto Key: Fixed
Keys will only be placed at the Auto Key Fixed Frame set in the field below the pop-up menu. Best used with static items that have no motion paths.
Note that the Autokey indicator at the bottom of Layout works in a similar manner to how the original Autokey on/off button did. For your Autokey preferences to be saved, you still need to visit the Options panel. This setting is not scene-specific.