I would switch to electric baseboard and make tenants pay for their heat via their electric bill . I don’t care if it costs 7 grand .to switch .,. It will pay for itself eventually
In my primary home we switched from oil to natural gas. Our furnace died so we decided to switch. We noticed a huge difference in heating costs. The gas company handled everything from running the new gas line to installing the new furnace. They even offered their own financing. I would call your local gas company and see what they have to say. A lot of times they have rebates/promotions going on.
You say the current furnace is old. It might be a good idea to replace it sooner than later because if it dies during the winter and you want to switch to natural gas, you'd have to wait until spring. They don't run gas lines in the winter and I don't think your tenants would want to use space heaters for 3-4 months.