I'm a bit confused about the circumstance. Are you saying that you can't work the shifts you were originally hired for? If you can't work the shifts they want you to work, it would be more to their benefit to get rid of you and hire someone that can work the shifts that they want. It just depends on your schedule... as long as you aren't scheduling yourself on days you had left open for your first job, then you shouldn't be losing it, since they would have no reason of knowing. But if you've changed your shifts or requested less time or different days, then there's a chance they may not want you around.
But I'm not sure... just my thoughts.