Originally Posted by
84fiero
http://www.law.cornell.edu/cfr/text/14/125.189
1) A demonstration must be conducted by the certificate holder upon the initial introduction of a type and model of airplane into passenger-carrying operations. However, the demonstration need not be repeated for any airplane type or model that has the same number and type of exits, the same cabin configuration, and the same emergency equipment as any other airplane used by the certificate holder in successfully demonstrating emergency evacuation in compliance with this paragraph.
(2) A demonstration must be conducted—
(i) Upon increasing by more than 5 percent the passenger seating capacity for which successful demonstration has been conducted; or
(ii) Upon a major change in the passenger cabin interior configuration that will affect the emergency evacuation of passengers.
It appears that a <5% increase would not require another demonstration, unless some other regulation requires it. Otherwise, another demonstration would have to be done. 90 seconds is the threshold.
The testing is generally done by the manufacturer (certificate holder), not the airlines. Testing is done at the max certified capacity of the plane. As airlines add seats, they have to stay below that capacity. To go above, the manufacturer would have to get the plane certified for more pax, including the evacuation testing.
As an example, the Airbus A380 certified capacity is 873, which is far more seats than any airline currently stuffs into one, but the testing was done with 873 people. There are some requirements about age and gender mix of the testers, so they can't all be 20 year old male track team members. The testing is also done using just half of the total number of exits.
Here is a page with some details about the evacuation testing for the A380.