Pay-as-you-go fees:
If you are paying fees as you go, then your next fees will automatically readjust upward or downward after you change your plan amount.
Annual fees:
If you paid an annual fee on signup and then later increased your Bitcoin purchases, you will pay a prorated fee to cover the difference.
Example: You prepaid a $25.74 annual fee (.99%) to cover a $50/week annual plan and then immediately increased your purchases to $100/week, you will be charged another $25.74 in prepaid fees. If you made that plan change after a six-month wait, then you would be charged $12.87 in annual fees to cover the remaining six months of your annual plan. 
If you paid an annual fee on signup and then later decreased your Bitcoin purchases, the portion of your annual fee that is unused is refundable for up to 30 business days. After 30 business days, your unused funds will be available as credit toward future Swan plans or purchases.