Retrospective application of a change in accounting principle is impracticable when
1. the costs of applying the new accounting principle to prior period financial statements are material.
2. retrospective application requires that management's intent in a prior period be assumed without independent substantiation.

Juanita and sam attend a beach party and notice that the local beach appears to have a great deal more trash washed up on shore than it did when they were young. the water doesn't appear nearly as clear, and there seems to be less evidence of small water creatures living in the shallows. an afternoon at the local library convinces them that one major cause is the new factory nearby. after some discussion, they decide their next step should be identifying the cause of the changes identifying the problem picketing the guilty factory lobbying their elected representatives to complain about the problem talking to a local environmental group about solutions