Help. Work issue...

So I really don’t know how to handle this. I work in a deli at Walmart. Not the best job, but better than my last. They hired me on to be one of the people that comes in early in the morning to make the fresh sandwiches. You have to be fast at it and it’s day shift so it’s considered a “privilege” I guess. They has 1 lady (we will call her Sydney) that’s been here like 10+ years that’s a sandwich maker and another lady names (we will call her Kim) that’s been here like 5 years as the other sandwich maker. My manager told me they want me to replace Kim as sandwich maker because Kim does it wrong even though she’s been trained and she isn’t fast enough. I didn’t think it’d be an issue because they just moved Kim to a slightly later shift. Well now Kim is all mad at me because she worked hard for that position and they give it to me straight away. She say’s I haven’t earned it and that it’s her right to have it because she worked for it for 3 years. Idk what to do. I like the shift and I want it. I don’t want her to hate me though. I also don’t get to pick, management does. Please help.