Most likely it's not shifting because the seals are leaking too much
when it's cold and the clutch packs aren't applying as a result.  If
it were mine I'd put a can of transmission conditioner in it, the kind
that softens and swells the internal seals.  It may help and it's only
a couple bucks instead of a couple thousand bucks to rebuild it.  I've
put it in half a dozen transmissions over the years and it's never
hurt them and often helped them.