I think that openness can be changed to some degree. However, even though I think that traits that are highly heritable are harder to change generally, I don't think one could say that one could say that the fact that openness to experience is 57 % heritable means that we have control over 43 % of our openness.

For instance, openness to experience is 57 % heritable in the present social set-up. This does not conclusively show, however, that it wouldn't be much less (or more) heritable in other social set-ups. For instance, it might be possible to develop techniques that increase openness to experience radically.

Conversely, non-heritable factors might be beyond our conscious control (as you indeed point out). A person with low openness to experience due to childhood traumas might have at least as hard a time changing level of openness as a person who is not so open to experience for biological reasons.

In general, I think, though, that high degrees of heritability signals that it is not easy for the individual to radically change the trait in question.