Establish what the home needs
Ask for a room-by-room heating and cooling calculation and the proposed heat pump's output at local winter conditions. PNNL recommends matching cold-climate equipment to calculated loads and relevant performance data. PNNL sizing guidance.
Then ask what supplies heat when the selected heat pump cannot meet the need, including any planned backup. That answer needs to be part of the design.
Compare two complete scopes
A plan that retains the furnace should identify the exact approved equipment combination, controls, changeover strategy, furnace condition, and necessary changes. Do not assume every existing furnace can accept every heat pump.
A plan that removes it should identify the replacement indoor equipment, backup decision, electrical work, removal work, and any remaining gas appliances. Ask a licensed electrician to assess required electrical changes.
Have the HVAC provider explain which existing components can remain and which require replacement. Request documentation supporting compatibility, rather than a general statement that the brands will work together.
Include costs beyond the outdoor unit
Compare installed price, expected energy use under your utility rates, maintenance needs, warranties, and remaining useful equipment. Include gas account charges if other gas appliances remain, and confirm any claim about eliminating them with the utility.
An operating-cost estimate should state weather, thermostat settings, and backup assumptions. A heat pump recommendation does not guarantee a lower bill.
Outage planning also deserves a separate assessment. Retaining a furnace does not establish that it will heat during a power failure; ask what electrical power and approved backup arrangement the proposed system requires.
Make the decision from the comparison
We would keep a serviceable furnace in the comparison when a compatible design meets your goals. We would consider removal when the complete replacement plan offers benefits you value and addresses winter performance, power needs, and total cost.
Your next step is to request both written scopes with the same comfort goals and load calculation. Choose the complete plan, rather than deciding from furnace age or a single equipment price.