Or, if you are feeling up to a literary debate with everyone and their grandmother, "irony." I wouldn't really call this an oxymoron, because that is usually considered to be a noun phrase instead of a statement.
I would just call it a paradox, because it is a self-referential statement that produces an apparent semantic contradiction.
But since it points to a truth by stating that contradiction it may simply be a form of irony.
We immediately understand from your example that nothing is constant, even though at face value the statement would seem to indicate the exact opposite. [The statement is itself a generalization, yet it points to a truth.] Nothing is impossible.
In philosophy or poetry, an antithesis can be used to oppose a first (thesis)...
In philosophy or poetry, an antithesis can be used to oppose a first (thesis) proposition.
An oxymoron is a figure of speech in which two words with contradictory meanings are placed side-by-side.
Writers have been using oxymorons for hundreds of years to express humor or irony.
Some oxymorons are so common that you might not even realize they're inherently contradictory.
Take "good grief." Grief isn't traditionally thought of as good, so the words are a paradox.