Yes, you can use hypnosis to alter perceptions of the past, but realize that it is only one technique to accomplish that goal. Social psychology principles can be used, for example, to alter memory.
A possibility for working with grief in the method of Bob and Mary Goulding from TA/Gestalt therapy whereby the person puts the deceased in the empty chair and lists Resentments, Regrets (Unfinished business), and Appreciations to the loved one, and eventually says, "goodbye," perhaps including a goodbye ritual honoring the precious memory.
Note: You do not need to be "hypnotized" into using hypnosis, but you may want to use the patient's beliefs and values if you deem it useful.
Good luck and keep us informed.