Okay, I really like this series even though we only have one chapter to go by. Could be pretty cute and interesting. Reminds me a lot of
"Shougakusei ga Mama demo Ii desu ka?" which has a somewhat similar plot of a mother (instead of grandma) dying and reincarnating. And it actually dealt with a lot of issues of reincarnation that are usually overlooked and ignored, which was pretty refreshing. If you like what you saw here whatsoever, I'd suggest checking that out as well.
My guess as to why a grandmother would start using the "ore" pronoun is because she stops giving a shit about being so formal and uptight. You see this all the time with older people were they don't give two shits about small conventions like that. And, especially in Japan where elders are so highly respected, few people are going to give them shit for it.