When should I replace aircraft and how do I do it efficiently?

the longer you keep a plane, the more expensive the engines become each month.

Also, when you try to change one aircraft's line (schedule) to another, keep in mind that some planes are faster/slower than others, and bigger planes require more turn time on the ground - so if you tightly schedule each line to have no more than 10 hours open, you may find yourself in a difficult position or have to not move over all the routes. So keep an eye on those hours.

In the sandbox, I'm doing a lot of what this person is asking. Upgauging B717 flights to B738/B739ER (yes, the 900 is worthless), or upgauging EMB 175s to B717s; B763s to 77W or 772's, etc. Just have to keep your eye on the # of flights.

And also, if you are going from one configuration to an other (such as C / Y to a plane with F / Y), you'll need to double check your fares as well.