One to One Correction - 401(k) Plan Uses Prior Year Method Otherwise Excludible

Plan has consistently been using the prior year method testing otherwise excludible employees separately.  In 2017 plan failed testing but did not do refunds in time so plan can not permissively disaggregate otherwise excludible employees to calculate the refund/contribution under the one-to-one correction.

Is the one-to-one simply a correction mechanism, or does it cause the plan to have a change in coverage for testing purposes because 2016 and 2018 are being tested on an otherwise excludible basis but not 2017? 

I could not find anything specific in the statutes or guidance, and appreciate all comments.