The "subject to change" allows you to change/raise a properly disclosed fee (say from $20 to $25 for XYZ). The "subject to change" does not give you the right to charge a new fee for something you didn't disclose. If you haven't disclosed a "returned item fee" then you would have to send out a notice to all existing loan customers that you will begin charging this fee on <date 30 days or more in the future>. Then wait the 30 or more days, then you can begin charging.
"Gratitude makes sense of our past, brings peace for today, and creates a vision for tomorrow." - Melody Beattie