I was wondering, how much work is it to retrofit a plane's panel, even with using modular instruments? I know they have to be tested but how complicated is it to install new instruments and probably also make a new panel to mount them in? Does it need to be tested or just inspected before the plane is legal again? I wondered this after seeing the difference between a old Rockwell Aero Commander 112 and a facelifted Commander 115 panel and while the basic shape was the same then the isntrumetns and buttons were arranged a lot better in my opinion, also interesting to see a plane with electric cowl flaps instead of a lever. Oh yeah that reminds me, is it possible to mount a different throttle-lever, for instance one that was bent slightly back so it's easier to reach? Or maybe some other knobs on the levers.
Frank
Well, I'm not a mechanic of any kind, but I can tell you what I know from my work here at Boeing, and having seen other projects done by people. Anyone with first-hand experience, please feel free to correct any misstatements by me please!
First, how much work is it to retrofit a plane's panel. ... answer... depends on the plane, and what the differences are in the new instruments being put in versus the originals. For instance, if there is going to be a new instrument put in that has different electrical requirements, obviously you have a lot more work ahead of you potentially. Same could be true of vaccuum based systems where hoses may be aged and in need of changing out, etc. If the physical dimensions of the new instruments are different than the old, it could also mean work on the panel's structure itself. As for inspections and re-certification, I'm not sure... so some A&P AI folks... please speak up!

I will say that one really can't easily do a mod where you change a steam gauge Cessna to a G-1000 panel. There's just too much of a difference between all the electrical systems, network, and sensors to make such a mod undertaking viable compared to just buying the plane that way in the first place. Future panel upgrades, however, on such G-1000 or similar systems is often going to be more a matter of inserting a new circuit card, and downloading new software, than physically changing out a panel component unless it's a Flight Directory input panel, or, a standard G-1000 DU with a newer right-side DU with AP built in for the MFD. Even there, it's "plug and play" on the new systems practically.
Is it possible to mount a different throttle lever... yes... but that too may require potentially extensive rework. On a Cessna, for instance, it might be a tougher job, whereas on a Piper, where the levers are interconnected differently with internal linkage arms, it might be an easier undertaking. I'd also be wary of changing the knobs on levers to any non-standard shapes. Part of a pilot's training is to be able to identify the type of lever by feel alone for the sake of safety. For instance, the throttle lever is usually a smooth round knob. A mixture control is usually a ridged red knob right next to the throttle. A flap control is usually a flat, rectangular switch, or a handle similar to an emergency brake on a car. And a gear lever is usually a vertically mounted disc that feels reminiscent of a tire... for obvious reasons. These kinds of standard shapes have come into use over the years, and works very well to help in contingent circumstances. In fact, during training, a flight instructor may give the prospective pilot the "blindfold test", where you have to indentify what you are touching by feel and placement alone.
Of course... given enough time, will, money, and resources... anything is possible
