Hi Wris,
No, the change is not automatic. You can put anything you like in the modifier because it's a free-text field; for example, a number, a phrase or, my favorite, the date when you entered the modifier. Using the date (any format) lets you know at a quick glance how long you've been using the same modifier. Just change the date (modifier) whenever you need a new password. You could also try a number and then just increment the number as often as you like.
HTH,
Eric