You can edit both hatch boundaries and hatch patterns.
If you edit the boundary of an associative hatch, the pattern is updated as long as the editing results in a valid boundary. Associative hatches are updated even if they're on layers that are turned off. You can modify hatch patterns or choose a new pattern for an existing hatch, but associativity can only be set when a hatch is created. You can check to see if a Hatch object is associative by using the AssociativeHatch property. (See the AddHatch method for more information on creating a hatch.)
You must re-evaluate a hatch using the Evaluate method to see any edits to the hatch.