  Charge-exchange rescattering $\pi^+\pi^-\to \pi^0\pi^0$ leads to a cusp
effect in the $\pi^0\pi^0$ invariant mass spectrum of processes with
$\pi^0\pi^0$ in the final state which can be used to measure $\pi\pi$ $S$-wave
scattering lengths. Employing a non-relativistic effective field theory, we
discuss the possibility of extracting the scattering lengths in heavy
quarkonium $\pi^0\pi^0$ transitions. The transition
$\Upsilon(3S)\to\Upsilon(2S)\pi^0\pi^0$ is studied in details. We discuss the
precision that can be reached in such an extraction for a certain number of
events.
