If, at the last moment, an O-1B beneficiary drops out and you need a substitute, you have no choice but to file another petition and seek an expedite. You should do the same for O-2 beneficiaries as well. However, there is a procedure to substitute someone not listed in the petition for a P beneficiary (principal and support), and on occasion, despite regulations to the contrary, consulates will use this same procedure for O-2 substitutions (particularly if there are multiple O-2 beneficiaries). In addition to the standard visa application materials, the petitioner must present a letter to the consulate, port of entry, or pre-flight inspection facility requesting the substitution and providing a copy of the original I-797. The letter should include the beneficiary's birth date, country of nationality, and position, and certify that the alien is equally qualified to fill the position described in the petition.