Get air to air heat pumps, and leave your current boiler right where it is. You will only use it on the coldest days. As far as moving forward, heat pumps are absolutely the future. Mine can physically heat until -19°f and they are going on 4 years old now. I will run my propane boiler in the single digits because the cost is greater to run the heat pumps at those temps. Bonus: you get central air and shoulder season heating is extremely cheap.

EDIT: I just read what you said about needing to replace your current HVAC system for rebates, and I have never heard that before in VT. I would double check that with efficiency VT and GMP.