Is my voluntary top-up to the 3 CPF accounts reversible?

Voluntary top-ups made are irreversible.

However, if the total mandatory CPF contributions and voluntary top-ups exceed the CPF Annual Limit in the calendar year, any excess voluntary top-ups will be refunded without interest. Therefore, do remember to consider your mandatory CPF contributions which you will be receiving for the calendar year, before making voluntary top-ups.

This information provided here is sourced from the CPF website.