I honestly would think rebuilding the current trains, complete gut and rebuild would be more cost efficient even if meant tearing them down to the shells and doing new running gear, paint, seats, ac, electrical and so on then buying new trains, I don't see them buying new trains unless they planned on enlarging platforms and making the trains larger or longer.
Do a Disneylandish rebuild, not retro, but a down to the bare bones and rebuild them type of deal, plus cameras and the such to improve safety at the same time.