Wat kind of sounds/effects are included with the original Protosound board? Does it do anything unique? (since it's a Millennium celebration type piece, does it maybe have announcements regarding such?)
I'll just throw out an example and people can laugh at it, but the Protosound 1 Shay had interesting Lumber Yard Proto-effects, and to the best of my knowledge that sound set would not be easy to duplicate with Proto2 or Proto3 (though I am guessing similar sounds may be commanded manually with the remote - buzzing saws, falling trees, etc.). Sure there have been many other road names made for Shays in both P2 ad P3, but there hasn't been a duplication of the way the Proto-effects work, to the best of my knowledge.
Subways were similar, but of course P2 introduced Auto Mode, which was (IMO) an advancement over the Proto-effects of the few Proto1 subway sets that were produced, so I'm not as certain I would consider changing a P1 subway to P2 or P3 a bad thing (aside from the concern over the Stations that are part of the announcements - I don't know if any P2 or P3 subway has ever identically reproduced the stations that were available on the first sets).