This came to mind as a follow up to the New-Old-Stock topic. I initially thought about this from an engine perspective, but realized as I was typing some might take a wider view.
For me, speed control (Odyssey, Cruise, etc.) has become mandatory for any modern command controlled engine, and I plan to upgrade via ERR or replacement any original TMCC 1.0 engines w/out speed control as funds become available. I'm still kicking myself for not getting EOB when I had the guys at TAS upgrade a modern conventional switcher and Berk Jr for me about 15 years ago. In the switcher's case, it's not a common livery (E&L), so I plan to keep it and upgrade it.
The Berk Jr is a different question. Adding cruise to existing TMCC still might run $250. It already has RS4.0, so no expected sound improvement. There's more, of course, that could be added like GRJ's superchuffer to get 4 chuffs and fan-driven smoke (I think), but that takes me very close to the cost of new LC+ 2.0 version which brings all of that plus better lights and sounds and a warranty.
Of course, the other option is to live with TMCC 1.0 as is. Nothing wrong with that inherently....I've kept all my postwar engines in their original, conventional operating state.
So, wondering what new modern features have caused you to replace an engine or other item with a newer version of same (e.g. replace TMCC with Legacy, replace PS2 with PS3) and was it worth it?