In a California divorce case, if you are already post-judgment ( in other words, you have a final divorce) and are asking for something from the court (for example modifying custody) you need to serve the other party, even if they have a lawyer who is the attorney of record and still in the case. More importantly, you have to establish they are still at the prior residence. The best thing to do to be certain is have them personally served the Request for Order.
Family Code 215
(a) Except as provided in subdivision (b) or (c), after entry of a judgment of dissolution of marriage, nullity of marriage, legal separation of the parties, or paternity, or after a permanent order in any other proceeding in which there was at issue the visitation, custody, or support of a child, no modification of the judgment or order, and no subsequent order in the proceedings, is valid unless any prior notice otherwise required to be given to a party to the proceeding is served, in the same manner as the notice is otherwise permitted by law to be served, upon the party. For the purposes of this section, service upon the attorney of record is not sufficient.
(b) A postjudgment motion to modify a custody, visitation, or child support order may be served on the other party or parties by first-class mail or airmail, postage prepaid, to the persons to be served. For any party served by mail, the proof of service shall include an address verification.
(c) This section does not apply if the court has ordered an issue or issues bifurcated for separate trial in advance of the disposition of the entire case. In those cases, service of a motion on any outstanding matter shall be served either upon the attorney of record, if the parties are represented, or upon the parties, if unrepresented. However, if there has been no pleading filed in the action for a period of six months after the entry of the bifurcated judgment, service shall be upon both the party, at the party’s last known address, and the attorney of record.