Input and output answer different questions
Input is the rate of fuel energy entering the furnace. Output is the useful heat the furnace delivers. BTU is a unit of heat; on a furnace rating, BTU per hour describes a rate, not a total stored amount.
A more fuel-efficient furnace can provide a comparable heat output with a lower fuel input. DOE explains AFUE as the relation between furnace heat output and fuel consumption over a season. DOE furnace-efficiency explanation.
Do not use the AFUE percentage as the entire sizing method. Request the manufacturer's actual output data for the proposed model and its applicable installation conditions. Seasonal efficiency and equipment selection have different jobs.
The old furnace is history, not proof of the new requirement
Ask whether the home has changed since the old unit was selected. Added space, insulation work, and changed use of the home are worth identifying. Also tell the contractor about actual comfort: whether the furnace kept up, cycled unusually, or left certain rooms cold.
Those observations help frame the assessment. They do not prove that matching the old size will be right. A furnace that was too large can still have heated the home, and a cold room can have a cause other than insufficient total heat.
ACCA's equipment-selection guidance connects the heating load with the manufacturer's performance information. ACCA Manual S.
Ask for the complete selection explanation
Request the load result, exact model, output at relevant settings or stages (lower and higher heating levels, if available), and how the selection fits. If elevation or another site condition affects the rated setup, ask for the applicable model basis. Do not use a generic Colorado adjustment.
Then ask about blower airflow, controls, and any existing or proposed AC match. A replacement should suit the whole installation, not only one heating number.
If two quotes list the same large BTU number, verify what each number means before calling the sizes equal. If a quote lists a smaller input, do not call it undersized until its output and home load have been compared.
Your next action is to ask the contractor to explain the replacement in delivered heat and documented home need. That gives you a meaningful comparison even when the old and new input labels differ.