Here the various social situations are to be understood as something Christ "assigned" to them at the time God called them to salvation.... The concept of "call" in the clause "as God has called" [v. 17] refers to conversion, that is, to their calling by God to be in fellowship with his Son (1:9; cf. 1:24). But the concern throughout is with their social situation at the time of that call, which is now to be seen as that which "the Lord assigned to each." That does not mean that one is forever locked into that setting. Rather, Paul means that by calling a person within a given situation, that situation itself is taken up in the call and thus sanctified to him or her. Similarly, by saving a person in that setting, Christ thereby "assigned" it to him/her as his/her place of living out life in Christ (The First Epistle to the Corinthians, NICOT, p. 310).And:
Paul wants them to live out their Christian life (i.e., their "calling" to Christ) in the situation ("calling") where they were when God called them to Christ. The emphasis is on both, that they can be Christians in whatever situation God called them, and therefore that they do not need to change situations - precisely because they are in Christ. Let their "calling" (becoming believers) sanctify the setting of their calling (p. 314).