While Flightplan allows you to change the aircraft name for any flight, there are many settings that may not be correct from one aircraft to another. Typically, changing from a “more complex” aircraft to a “less complex” aircraft (for example from an A320neo to a Cessna Skyhawk) will not have any noticable change when loading the flight plan in MSFS after changing the aircraft. Although switching from jet aircraft to propeller aircraft or from an aircraft with a single engine versus multiple engines can cause the aircraft to start “cold and dark”.
Flightplan preserves ALL settings in a flight plan, but only the settings displayed within the flight plan application can be modified. Commercial jet aircraft in particular have many, many settings related to aircraft systems and engine parameters that are simply not relevant to a light propeller aircraft, so MSFS will generally ignore the settings that don't make sense for the aircraft. But if settings are missing that it needs to initialize the aircraft, it will default to "off". So while you might save a flight plan with a single engine propeller aircraft and later change it to a commercial jet in Flightplan, the engine parameters for the prop engine will be incorrect for a turbofan jet so the engine(s) may be shut down when the flight plan loads.
If you want to be certain that particular aircraft systems/settings are initialized when the flight plan loads in MSFS, use MSFS World Map to save the flight plan with your desired aircraft, then open it in Flightplan. You can modify the route, ATC settings, fuel loads, radio frequecies, etc. in Flightplan and it will save your changes AND preserve all of the engine, propeller, avionics, etc. settings that were saved by MSFS. You can then save multiple copies of the flight plan with any route you choose and the aircraft settings will remain the same in each. Since there is a virtually infinite combination of parameters to support every possible aircraft that exists (or may exist in the future), Flightplan does not change any of the aircraft specific parameters, but it does preserve them when saving other types of changes (route, ATC, etc.).
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