@snood,
The first thing is who "they" is, exactly.... I think the bloc of people who supported Bernie in the primaries is pretty diverse.
Many of my friends on Facebook supported Bernie strongly for a while but they didn't hate Hillary, they just liked Bernie better. That subgroup started to peel away pretty early, when it became clear that barring something catastrophic, Bernie was not going to win. That group supports Hillary already.
There is another (smaller -- only one person I can think of that I have a direct connection with, though like-minded friends of hers chime in when she posts something) group who is just 100% anti-Hillary, period. They're saying that they don't give a damn what Bernie says -- that he was the result of a movement (not the cause) and the movement will go elsewhere if he tells them to vote for Hillary. They're talking about Jill Stein a lot.
Then there is a middle group -- also midsize out of the three, in my own sample -- who really liked Bernie, really dislike Hillary, and have just kind of gone quiet in the last month or so. When I see stuff from them it's more anti-Trump. I think they are going to end up as Hillary supporters but they want a bridge, a way to do it. I think this is the group that would be most influenced by what Bernie says and how he says it, but not necessarily in terms of whether he changes their mind in any way, but in terms of how good of a bridge he gives them.
I think he'll give them a good-enough bridge.