Corn sugar and cane sugar are quite a bit different. Pound for pound, the cane sugar gives more gravity
points(which converts to more alcohol) than the corn sugar does. The cane sugar is a little sweeter. Corn sugar ferments
a little more cleanly and leaves less of a winey or cidery taste than the cane sugar. And finally, cane sugar is a little
more economical than corn sugar. More to the point of your question though, you can substitute either one, but I use
cane sugar in my wines that require added sugar.
Now, if you add too much sugar, just by a little bit, then there is probably no big problem. If you say,
doubled the sugar, there may be some problems with getting the ferment to start or to finish dry. To fix the problem there
you might just dilute.
Typically on all malt extract beers with standard malt extract you should expect 75% fermentability. That
means the 75% of the original gravity will be fermented out and your final gravity should be 25 % of the original. This
changes greatly if you are using a dutch extract (much less fermentable like 55%) or using refined sugars like corn sugar,
cane sugar, honey, rice syrup solids. These sugars are 100% fermentable and will not leave anything behind. They will
contribute to the original gravity but subtract from the final gravity.
What I do to figure is something like this. Add
up the numbers for the original gravity and then subtract the numbers from the completely fermentable adjuncts. Use the
remaining number and take 75% off that to reflect typical final gravity. This being said, it is only a rough gauge of expected
terminal gravity, as it is a pretty complex issue. The other variables would be yeast strain and their attributes and condition,
conditions in the wort such as available oxygen and original gravity, and composition of the malt extract itself, as it can vary
from production run to production run (sometimes significantly).