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. In October 2010, the State Department also revoked the ability to substitute P support personnel. On occasion, consulates will use a procedure for O-2 substitutions (particularly if there are multiple O-2 beneficiaries), and some posts still grant P-1S and P-3S substitutions. When faced with a need for a substitution, contact the consulate to see what might be possible. In addition to the standard visa application materials, the applicant must typically 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. The letter must be printed on petitioner’s letterhead and signed by petitioner.
For reference, below is the guidance regarding the substitution process, directly from the U.S. Department of State Foreign Affairs Manual:
9 FAM 41.56 N8.8 Substituting Beneficiaries
Beneficiaries may be substituted on P-1, P-2, and P-3 petitions for groups. It should be noted that all groups qualified for P status may benefit from the substitution procedures. The petitioner must submit a letter requesting the substitution, along with a copy of the petitioner's approval notice Form I-797, Notice of Action, to the consular office where the alien will apply for a visa or the port-of-entry (POE) where the visa-exempt alien will apply for admission. The petitioner must state the alien's date of birth, country of nationality, and position, and must certify that the alien is qualified to fill the position described in the approved petition... You should note that essential support personnel cannot be substituted. In order to add different essential support personnel to an existing P visa petition, a new I-129 must be filed at the appropriate DHS service center.