Cal. Pub. Util. Code § 24356

Upon 10 days notification of the parties concerned, the department may reduce, or, within the limits specified in Section 24350, increase the amount of security ordered in any case if in its discretion the amount ordered is excessive or insufficient. In case the security originally ordered has been deposited, the excess shall be returned to the depositor notwithstanding the provisions of Section 24357. Substitution of security shall be permitted.