1. Taxable gross weight increased
You reported a 60,000 lb truck and then started hauling heavier loads, and now it's a 72,000 lb truck. The truck moved from category F to category R. You owe the difference for the rest of the year, prorated from the month the weight went up. File by the last day of the month after the increase.
2. Suspended truck exceeded the mileage limit
You filed the truck as suspended (under 5,000 miles, or 7,500 for farm trucks) and it went over. The full tax is now due for the year, prorated from the first month it was used, not from the month it crossed the limit. File by the last day of the month after you crossed it.
Not an amendment: a typo in the VIN (that's a VIN correction), or a truck you sold (that's a credit on Form 8849).
Amendments are included on the Plus and Pro e-file plans. See plans.