Originally Posted by IsleOfMan

I'd say from experience that far fewer commercial aircraft engines are replaced than you would expect. The JT8D is overhauled every 10,000 flight hours (baring some catastrophic failure), which is once about every 2.5-3 years. With proper overhaul and maintenance these engines can easily see 30 - 40 years or more of service life. There are extreme cases in which an engine will be swapped but the engine pulled is rarely written off or scrapped. There are also some instances of fleet-wide engine upgrades (such as the DC-8s flown by UPS being upgraded from JT3Ds to CFM56-2s), but this is an even rarer occurrence. There are thousands of B727s and DC9s still flying with older JT8D-5s, -7s, -9s, -15s, and -17s, all of which are older than the -200s flying on the MD80s, some of which are likely approaching 40+ years in service.

I know AA at one time discussed w/ MD a re-engining of the MD80s with either V2500s (from the MD90) or BR715s (from the then announced MD95), but this plan was scrapped when Boeing purchased MD.