It's really annoying trying to write a 3 when you wrote a 2. You have to scribble out the 2, because it looks really crap when you try to put the 3 over the top. Sometimes things are ok - turning a 2, 3, 5, 6, 7 or 9 into an 8 is easy enough, as is 1 to 7, or 1 or 7 to 9, and you can almost get away with 7s into 4s if you've not made the top bit too big. But don't even try turning anything into a 1 or a 0, unless it's a 1 into a 0. And 8 can't become anything, and nothing can become 1. The whole thing would be easier if I was incapable of writing the wrong number in the first place...

(disclaimer: the above list may or may not be exhaustive. The author bears no responsibility for people wasting time/paper/ink/graphite trying to turn numbers into other numbers without making them look crap)