Originally Posted by
Sailvi767
Jet engines usually get overhauled not replaced.
It's quite common for the engines on a high time aircraft to actually have more operational time (since new) than the airframe - I've seen engines that were approaching 100k hours (obviously overhauled several times).
New run engines are typically good for between 10,000 and 20,000 hours before overhaul - short haul, high cycle engines tend to the low end, long haul/low cycle engines toward the high end - sometimes north of 25k hours before the first overhaul.
Overhaul doesn't return the engine to 'like new' - so after the first overhaul, subsequent overhauls come more frequently, roughly half the time between overhaul of new production engines.
As DR noted, retrofit re-engine programs need a huge improvement before they become cost effective. What are common are incremental improvements - things that show up in new production engines become available for retrofit into older engines during overhaul. Such improvements are generally small - a quarter percent here, a tenth percent there, but add up. The CFM56-7 engines being produced now have several percent better fuel burn than the ones produced 20 years ago - made up a dozen or more small improvements. But it's hard (and usually expensive) to squeeze those improvements out of an existing engine. Rolls will be hard pressed to provide the rumored 2% improvement promised - more so since they're only looking at a market of a few hundred engines to spread the costs (CFM had the luxury of spreading the improvement costs over thousands of engines)
But if you want a big improvement, you need a new engine design - and that's a lot of money.