what is the difference between "used to" and "would" ?
These two are both used to say "something happened in the past". For example,
1. Looking back my childhood, I used to(would) swim in the creek close to where I lived.

are "used to " and "would" interchangeable in the above sentence?

what is their difference?

Question

what is the difference between "used to" and "would" ?
These two are both used to say "something happened in the past". For example, 
1. Looking back my childhood, I used to(would) swim in the creek close to where I lived.

are "used to " and "would" interchangeable in the above sentence?

what is their difference?

Answer

In some situations, such as your example, it's possible to use either one. I'm guessing you've already found the rules which explain when you can only use one and not the other.

When "would" is used to mean a habit in the past, you can understand it as a *predictable habit* ("will "for predictions remember, and here we have the past form).