OWTS,
How feasible is replacing the ride mech within that building?
Personally, I can't imagine (just from an engineering standpoint) how to replace the ride mech without bringing down the globe (or do you just mean replacing the track along the current path? That might be doable, if expensive).
I'm also concerned about the condition of the RCID jurisdiction in modern times. I just don't think Spaceship Earth could legally be built today, even within RCID. That affects how much you can change an existing structure, as well as new construction.
From the things I know and the things I'm pretty sure of, I'm comfortable saying the following: AT&T is gone and Microsoft is waiting in the wings; but the specifics of the deal are cantankerous, as the SE building will not support the kind of upgrade Microsoft felt was its moneys' worth, compared to M:S.
I personally doubt there will be any significant change to the SE building or ride path until the whole thing Horizons all over us. A re-write, new scenery and perhaps new track/train over the exisiting ride path will be done just as fast as MS is convinced to hand over the bling-bling.