what changes are necessary to convert the 2,7L sebring to a 3.0L or 2.4L?

It will cost you more than the car is worth. Trade it or sell it.

you need to change computer and possibly some plugs depending on what wiring harness you have you will also need your old power steering pump and alternater may have to wire old plug to 3.0 or2.4 alternater so make sure to get alternater plug off your other engine (meaning the engine you are going to put in) and any other plug for engine electrical components such as a/c compressor o2 censor heat gauge cam shaft damper or any thing electric