Furnace & AC blower unit

[Matthew Defore]

I have two properties with furnaces and AC outdoor blower units that are approx 20 years old.  They are still performing at this time.  Should I wait until they fail before replacing? Or should I replace them now before HVAC technicians are in high demand during the summer (mississippi) ?  The only consequence of letting them fail that I can think of, is that the tenant will be without heat/AC for the amount of time that it takes to replace.  The obvious consequence is that I might otherwise be able to defer these costs another year, two or five if I ride it till the end. 

Your thoughts? C-class properties in a college town FYI

Matt