If I change a Helper layout, will it change where it's used?

No. Helpers are coupled with the page once placed. Changes to the Helper layout will not affect existing instances.

Helpers allow you to save Elements or groups of Elements on your Layout canvas to be used again quickly without having to rebuild them. You can think of helpers as an advanced copy and paste function. When you save a Helper it saves with it any child elements.

When a Helper is added to the Layout canvas it turns back into Elements. If you change the Helper, it will not affect any Helpers that have already been added to the Layout canvas.

For more detailed information, see Creating and using helpers in the user guide.