Ok, yeah, you really do need an A to transfer into CS Engineering, and almost as high a grade for CS+X majors. That can be a problem.
Right now, it's very VERY hard to transfer into the CS major. No one likes the high grade requirements, but we don't have the resources to admit everyone that would do well in the major. So some perfectly reasonable students are not going to get in. Sadly, that may include you.
The CS department hates this situation and is trying to get more resources. However, better resources probably won't appear fast enough to help students currently trying to transfer.
Do as well as you can in CS 173 but have a backup plan. If you are in another major, stay on track with its requirements. If you are in DGS, take the beginning courses in at least one other major.
Don't stress out over whether you will or won't get the A. Stress makes you perform less well. Try to put the grade issue out of your mind, as much as you can, and concentrate on doing your best.
It is not the end of the world if you don't get into CS at UIUC. Many schools with a somewhat lower ranking are quite good places to do computer science. Many jobs (even research jobs) benefit from expertise in CS plus another field. You can do well with another major plus a CS minor. A lot of CS researchers and faculty members who did something else as their undergraduate major, or went to a school outside the top-10.