If you are writing fanfiction you can't just change how a character reacts to things immediately. You have to show them changing or have a kind of backstory. You can make them different to what they are usually like to help your story but you can't make it like they have a completely different personality.

If you are changing a characters personality to help with your story it gives you more to work with. For example to me Dramione fanfiction is amazing because the writers have so much to work with and there are so many different storylines you could create because the characters hate each other.

Remember that this is just my advice and you can do whatever you want with your own story, I am just trying to help you so you don't change to much to quickly.