Not an issue.

This should return null, because there's 10 in array.
I think the array for test case should look like this: Test.assertSimilar(upArray([0, 9, 9, 9]), [1, 0, 0, 0]);

invalid sequence is a string with non numeric character. description updated, please check if it's clearer.
thanks!

It doesn't. That's what your solution returned.

I am a bit confused. How does [9,10,0] represent the number 999? I can see, I guess that the 10 can be converted to a 9(?) but why would it not be 990 then?

999 + 1 = 1000
Alhough the description isn't exactly strict, it looks clear enough for me.