You can install non ducted mini splits, which is what I did. I left my oil furnace intact and added 4 high wall units (each bedroom and living room) with 2 out

You can install non ducted mini splits, which is what I did. I left my oil furnace intact and added 4 high wall units (each bedroom and living room) with 2 outdoor condenser/compressors. I still use my oil furnace when it gets below 40.