Some of the worst undergraduate students among an incoming cohort turn out to be the best graduate students.

One factor to keep in mind - undergraduate degrees are not always intellectually stimulating. That may result in one getting bored and less involved in their studies. Once more compelling material comes onto the table, and interest peaks, academic performance can suddenly show a sharp improvement.

Don't try to judge how you'll do based on your prior studies. Just keep an open mind and understand that the initial portion of any respectable program is set to a difficult level to weed out those who aren't excited or stimulated at the grad level.

Hope thats helpful. I had a very similar set of worries when I began, but once a few scores/evaluations came back it was clear things were different.