How do I convince my younger sister to trade me rooms?

You can't really blame her for not wanting to switch now when you didn't want to before. She's probably just decided to make the most of her small room. I understand that it makes sense to swap now though. Why not let her see how big it looks and talk about all the great sleepovers she could have and how much more entertaining she could do. Make sure it looks cosy still as she might not like the idea of an unwelcoming room. If you tell her that you want to change because it makes no sense for you to have the "better" room if you are never there, then she might believe that you are doing it for her too and won't be suspicious. After all, if the room is so great, why are you so keen to leave it! Explain that you have less stuff at home now so it would be better for her. Good luck!