Unusual line of reasoning in Kant's response to the antinomy of free will

In response to determinism, Kant explains how the antinomy of free
will arises from a misuse of reason:

"The possibility of an infinite origination, without a first member in
regard to which everything else is merely subsequent, cannot be made
comprehensible. But if you want to dismiss this puzzle of nature, then
you will find yourselves compelled to reject many basic synthetic
characteristics (basic forces) that you are just as little able to
comprehend, and even the possibility of a change as such must then
become objectionable to you."

Up to here, I followed him. But then he says,

"For if you did *not* find through experience that change is actual
[my emphasis on "not"], then you would never be able to excogitate a
priori how such an unceasing sequence of being and not-being is
possible."

If instead of "for" he had written "however," I wouldn't have
considered it unsound writing. But then I would need a definition of
"change as such."

I would have expected a proof by contradiction, i.e., "If you *did*
find through experience that change was actual, then..."