If that your cursor is on the left brace that you want to change:
If your cursor is on the right brace:
Basically, ask vim to find the matching brace, change it, then jump back to the first brace and change it too.
If your cursor is near enough to one of the braces you want to change, then you can do much as the above but double the initial %:
This works across multiple lines, and with nested brackets.
You may even find it works with xml or html, depending on various things.