Granddaughter wants to change her name.

When I was around the age of ten one of my friends, same age, announced that she no longer wished for be called Lillian. She wished to be known as Pat. From that point on, Pat she was. She has gone through life known by the name she chose for herself. So it's quite normal. I wouldn't worry.

A lot of people go by their middle name. Her aunt changing her name has just probably made 10 year old aware it's possible. Why not?

I still don't like my name. I've wanted to change it all my life but didn't want to hurt my DPs. Now its really a bit too late.

My DF and his brothers were all called by their second names so I don't see it as a problem if this GD wants to do the same. It's still a name her parents choose. (I didn't like my second name either!)

My brother chose to be known by his middle name when he started his first job as he had always disliked his first name.

Close family carried on using his given name. It was very confusing when everyone got together for his wedding as at times it seemed we were talking about two different people!

Yes, I would treat this as a normal, and not uncommon thing. Certainly I have known lots of people who have done this - sometimes it sticks, sometimes it doesn't.
It is inevitable that you and others will forget and use her given name from time to time, and you will see by her reaction how important it is.
I would also say that she should try it out among friends and family before school is formally told.
Cultures vary a great deal with given' and 'chosen' names. For many of us born into native British families, 2 given names often meant that one was liked by the parents, and one had a family connection.

Perhaps she has never liked her first given name. Learning that her aunt has changed her name (the circumstances are irrelevant imo) has just alerted her to the fact that it's possible to call oneself something other the name others have always used.

She's opting for a given name not one chosen from elsewhere so it simply sounds like she prefers her middle name.

Ten seems a good age as when she goes to senior school next year she will be known from the start by the name she prefers.

From the age of eleven, I had a good schoolfriend I always knew as Susan. We were 50 before I learned that was her middle name and she didn't like her first.

Back in the time when children were named generation after generation after their parents it was common for them to be known by their middle names to avoid confusion. Not the same thing I know but nowadays when it's more likely not to be named after a parent or a grandparent, one wonders what the purpose of having two or more given names is.

Go with the flow. If it’s her middle given name, why not?

If someone says they want to be referred to by a specific name, surely the only polite thing to do is to call them that.