On Fri, 17 Jan 1997, Zen wrote:

Anou...  MAYBE.  Y'see, it was never established that Shampoo's instant
Nanniichuan was from Jyusenkyou...  Could be an entirely different
mechanism.
However, the additive properties of *Jyusenkyou* springs ARE positively
established, as evidenced by the aforementioned Pansuto Tarou...

Not necessarily.

You could argue that Tarou's octopus spring _did_ replace, rather than add.
He just put it on only part of his body, and the curse on that part of his
body was replaced by octopus tentacles instead of whatever it was before
(probably fur).  Unless he got fur-covered octopus tentacles, this doesn't
mean that curses add.