Just learned this, kind of interesting to me. If the column contains data I can increase the scale but I can't decrease it afterwards to its original scale, even though the numbers would fit. Also from what I've read the only case to decrease a scale is with set to nulls, delete, or maybe copy the column into an appended column.
Just think it's a bit odd you can't decrease a number column even though the values would fit.
Does this have something to do with leading zero's?