I think Davis should be tried at 3B, regardless of what he prepared to do. Here's why:
a) we have a problem at 3B right now - it's not just a question of cultivating future talent, we have no one besides Davis who can do an adequate job there.
b) the position isn't new to him. He's played hundreds of games at 3B in the minors as well as 69 in the majors.
c) his bat is far more valuable to us as a 3B
I don't think "fair" is an issue here - it happens all the time that a player who expected all winter to be at Position A is told he will be at Position B.
I'd still be hesitant to make that move, but the better offensive start that Davis' gets off to, the easier it would be to consider moving him across the diamond. If he was struggling with the bat, I wouldn't him to also have to focus on switching positions.
The official reason given for not sticking with the Davis @ 3B experiment from last season was his shoulder won't be able to withstand playing there all year. I don't know how honest that explanation is, but it has to be given some credence.
I would stick with Reynolds at 3B for a little bit longer, but if we decide to try something new over there, I'd be fine with giving Davis a shot if they think he can hold up physically.