In general, I find that when using an OO approach with a granularity that is not too large I seldomly run into problems when I want to change the code later.

Classes should map as closely as possible to the "objects" involved in the problem you're trying to solve (that's what I mean by the "right" level of granularity).

Altering functionality can be done by using the typical OO techniques like inheritance or composition.

For me, the threshold to introduce classes is very low, but opinions on this topic differ widely.

Just my 2 cents, -gjb-