I used to think that having a calling means having predicted what will you reach at the end of the line. That it is sealed for the rest of your life. We want to think that way because we want to make sure everything is under our own control.
Most of the times, it is not the case. God might call you for one goal for a certain time but He might call you for other goal afterwards.
Another thing is, sometimes we are to focused to our calling instead of The Caller himself. We are too busy achieving our goal we forget God might have different saying with what our doing. I have to admit I also made the same mistake.
The most important thing is how we start or how we end, it is at the very moment whether we remain faithful to Him regardless our best or lowest point.
Having a calling doesn't have to be working in the same area you have passion about. The scoop of calling is sometimes larger than your job. You can still do your job but you can serve on the other area outside your job whigh you also have passion on your spare time.
Another point is about the different definition of work and job. In christianity, working means we become blessings for others but also we fullfil our need and passion, all directed to glorify God. These are inseparable aspects of work. Having a job, on the other hand, speaks more with having things to do so you can have some income, not necessarily something you have passion about.