Should all motor mounts be changed if one is broken or worn
I wouldn’t, unless they were disconnected as part of the repair. But I would inspect the others carefully.
Why fix something if it ain’t broke?
More to it than that. Motor mounts contain a lot of rubber for noise and vibration control. As all aromatic hydrocarbons, rubber in motor mounts will out-gas and become more brittle depending on time, temperature, and humidity. Much like replacing timing belt can be more time dependent (4 to 6 yrs) than mileage dependent (60 to 80k). Expense to replace all motor mounts not that much more than replacing one. If dumping vehicle in next few months -really want to burden the buyer with a surprise?
Just the one that’s worn.