Hmmmmm. A thought that I have is that even if these two locomotives need a lot of work (putting it mildly), at first glance there are big positives for both.
For the D&S with 483;
They get a locomotive that they have shown an incredible ability to rebuild and restore to operation. Remember that 482 was called a basket case yet the shop forces had it up and running in 6 months. I know that 483 is missing more pieces yet D&S has an incredible shop that has only increased in its ability. Finally, though the friends have done amazing work cosmetically, she would realistically never run again if left in Chama.
For the C&TS with 478;
They get a locomotive that first and for most, the type has not seen any of the remaining 64 miles of railroad since 1968. Even if she sat static for a while, people (ok, foamers) will come just to see it in a once plied area. The same will be true for 168. As for funds for restoration, I will point to the 463 as an example. Despite a decade plus of trying to raise funds for 483, she barley has raised enough for her much needed face lift, yet in that same time, over 1 million was raised for 463. I believe this has many reasons, but mainly revolve around the fact that 463 was the only type on the line. She has a surviving sister, but she is not close by and not on that line. This gives extra fundraising ability and focus for many target areas for funding that would decline when others are already running on your rails (484,487,488 and 489). Using the "unique" label will help draw from additional resources. Even when you are looking at two such projects at the same time. It also helps with the moniker of "living museum" to have different examples. This also might go a long way to helping funding of a rebuilt round house.
If the respective locomotives stay where they are currently, the 483 will continue to sit dormant with the continue need of resources just to keep her in the state she sits in today. This is not bad, but means it will not contribute to bringing in revenue or publicity to the railroad. The 478 is now the second of three 470s in "long term storage". Best case I would guess is that eventually, if the need was there, that the 478 and 476 would be combined to make one locomotive. As I understand, the main issue with the 476 is the cracked frame. To fix it the boiler has to come off. So perhaps combining the two for one gives them what they need as well is cheaper than a complete rebuild of just one of them. It is my perception and after talking to several close to the D&S management, they like the 480s and only feel that they need one smaller locomotive for specials and back up. I even heard that some really are not a fan of the 470s.
In conclusion, a switch allows both to have some gains in the way they do business as well as the short lived publicity of the locomotives being moved. Remember the PR for the 482/497 swap? Even the 486/499 made headlines and who knows if this too does not help draw attention in a positive manner. Granted, the moves themselves cost money, but you would be surprised what monies are available for these sorts of things but will not work for the status quo.
Phil