If a creature has a minor change, but then changes back, how could it ever accumulate enough changes to become a new species?

By not changing back.

Not a very scientific assumption.

Explain.

You would need to test or study on what is the basis for permanent adaptions and temporary adaptions. What are they, how do they come about, what are their uses or benefits from what was before? What are the odds that multiple changes could happen in a species? Could these changes cascade? At what point would we consider a creature to no longer be of the same species? When it could no longer mate with others of its original species? There are too many questions, dependent variables, and unknowns to simply assume that the adaptions would simply not revert.