If your water is not highly corrosive and the plumbing is reasonably accessible, and not leaking, I would leave it alone.

The downstairs unit is currently on oil and the system is currently functioning but the tank has corrosion and the furnace is in rough shape.

I don't know how old your lines are but we discovered corrosion buildup in our 3/8 supply lines within the shutoff valves. 

Like anything else they have issues mainly corrosion.

Bird poo is very corrosive on paint work.

as a plumber in MA, I'd check under every sink specifically looking at the water shutoff valves/piping for corrosion, that is a sign they will leak soon enough.

All visible metal connectors are: Secured to truss/rafter with a minimum of three (3) nails, and Attached to the wall top plate of the wall framing, or embedded in the bond beam, with less than a 1/2" gap from the blocking or truss/rafter and blocked no more than 1.5" of the truss/rafter, and free of visible severe corrosion.4. 

I like to look at my body like a machine - if there's a constant flow of corrosive acid and the gears aren't turning nearly as often as they should, I, as a machine am not a very good one. 

What usually happens is that the problem points that are most susceptible to corrosion go bad, you replace them, and the system works just fine for decades on decades more.What you need to make sure of, though, is that the sewer line to the street is uncompromised.

I've listened to enough escapist fantasies to learn to hate all the empty, vapid, corrosive, soul-sucking dreams of something for nothing attached to those dirty little tickets.This thread has served to remind me yet again of how few people come from a broke childhood to real estate investing as a way to build lasting wealth and find financial independence for their families, of how incredibly fortunate I am, and how grateful I should be to those who have helped me along the way.