What is the best way to tell Grandma with dementia I’m moving?

I’ve been assisting (part time) with my Grandmother’s care since she had her second stroke last year. At first, I was with her every free moment I had. Since the beginning of this year, all family members involved have cut back (for our sanity) and relied more on paid caregivers. Anyway, I left an extremely stressful job in March and now accepted a dream job in another state over 12 hours away. I’m not sure how to tell my Grandmother I am moving. She has been diagnosed with Dementia and her short-term memory is practically gone. I know that I need to tell her, but I also know that she will forget and need to be reminded which will upset her all over again. I could wait until the last minute, but I don’t think that is right either. I guess it would be easier if my grandmother had always been the loving, supportive type, but she hasn’t, and despite her memory deficits, her personality remains and she will take this move personally. At the end of the day, it is all about her. Any advice on how to talk to someone with Dementia about moving and life changes in general?