I have had a similar letter 'offering' to increase my direct debit from £21 to £36 a month [I am a low-usage consumer], and my last two bills had a credit balance - but they still take the monthly payments as the scheme is designed to even out the cost of a year's energy over approximately 12 months.
Unfortunately, they calculate this new payment based on their estimate of my next year's energy usage according to their letter, but do not appear to take into account correctly all of the following items-
1. their dual fuel tariff discount
2. their discount for payment by monthly direct debit
3. their discount for using an online account [no paper bills]
4. accurate readings by meter readers, so no estimated usage or charges to upset the calculations
5. billing accounts or an individual bill which have a credit balance, so no payment is actually due for that bill
6. other credit/debit adjustments to bills including when a bill is amended, re-issued or readings are obtained
7. the specific regulations which apply to energy bills and monthly payments for a company such as E.ON
I do not think I need to increase my payment to any more than £25 at the most, so I will be contacting them using the specific phone number to request them not to take the revised amount of £36 as I do not see my charges increasing by almost 72% even with the tariff of increased charges recently notified to customers taking effect for the next year. You may have a specific phone number for your situation - so you could call them and see what they can do.