I've heard from a number of different sources that after 489 the next two done will be 463 and 497. It would seem logical to me that 463 would be next as it is lighter and less rough on the track while 497, having more power is acknowledged to be rougher. For this reason they might hold back a while on it until the raodbed is brought up to a higher standard.
It might be possible that the first heavy rehab on track could be done from Chama up to Cumbres and then 497, when it is finished could be used as the helper up the hill on a regular basis.
Also, since 463 is stored in Antonito, and was a gift from the City, it might be possible to do some of the rebuild work there as was done with 487. Playting a little politics here would not hurt.
It would obviously be expensive to work on 463 and 497 at the same time and still do basic maintenance work on the others but who knows.