Obama didnt change it, Congress did.
If the NMB determines that a single transportation system exists, the investigation will proceed to address the representation of the proper craft or class. The rules regarding percentage of valid authorizations in NMB Rule §1206.2 (29 CFR § 1206.2) and bar rules in NMB Rule §1206.4 (29 CFR § 1206.4) do not apply to applications filed under this section.
19.601 Showing of Interest on the Single Transportation System
Incumbent organizations or individuals on the affected carrier(s) must submit evidence of representation or a showing of interest from at least thirty-five (35) percent of the employees in the craft or class. This evidence includes, but is not limited to, a seniority list, dues check-off list, a current collective bargaining agreement or a certification, or other indicia of current representation.
19.602 If the organization or individual filing the application does not represent any of the employees covered by the application, the organization or individual must submit authorization cards from at least thirty-five (35) percent of the employees in the craft or class. If a single organization or individual represents all of the employees in a craft or class on the newly created single transportation system, an application filed by an intervening organization or individual must be supported by authorization cards from a majority (more than fifty (50) percent) of the employees in the craft or class. (See Section 3.601).
19.603 If not already filed with the initial application, organizations (Incumbents and Intervenors) have fourteen (14) calendar days from the date of the NMB’s single transportation system determination to submit evidence of a showing of interest or to supplement the showing of interest on the single transportation system.