Is there a way to change the text of all existing "next" buttons in a form?

You can change the "Next" button text in the Page Break Properties. You need to do it one by one. Here's a screen animation guide:

1. Log in to your account, select the form and click the Edit form.
2. Select the Next button and click the Properties button.
3. In the General tab, you can change the text of the Back and Next button of your form.

I hope that helps. If you have questions, please let us know.