For Sochi, all skaters going to Nationals had to tell USFS in December 2013 in writing whether they wanted to participate, and, if so, in which program(s), and if they'd reconsider if the team needed them to do something else. As if they would appear to be rigid before Nationals was judged and the selection committee met.
Then USFS ranked the disciplines in the order of strength. The top skater/team in the strongest discipline chose first, in theory. If they didn't want to do both, or did, but were strong-armed into sharing, they get to do their preferred program unless they were strong-armed into doing the other, and then the next strongest skater/team, if there was one, would have been offered what was left, and if that skater/team didn't want to skate at all or the program that was left, they go to the third qualifier, if there was one. Then the next strongest discipline, etc., until the team was filed, with a max of two disciplines splitting segments.
In theory, if there wasn't a full roster, USFS could have gone back to skaters who agreed to consider changing their minds, and if they didn't change, to compete with three disciplines or give back the spot if there were three or less.
The feds don't have to declare subs for the frees until after the last short is competed. I don't recall if USFS had to tell the skaters earlier if they are subbing, or if it just maked strategic sense to announce right away and to prepare them.
In Sochi, D/W, like V/M, were the top team in the strongest discipline, and they had the clout to choose to skate both. I don't think any US skater or team has the clout to do that this year. With a tight race with China anticipated for team bronze, especially since it's a likely scenario that US will have to make up a pile of points in Pairs, USFS might prefer Chen to skate both Men's segments. I'm not so sure that would be Arutunian's preference.