In this link from "DWC - the 401(k) experts" they spell out what I believe is the standard industry interpretation and what I believe has been explained by the IRS informally, for this exact situation. I'm not citing them as the ultimate authority but they didn't make it up either; this has been repeated many, many times and I didn't think there was really any debate about it. My emphasis in bold.
Profit Sharing Allocation Methods
From time-to-time, a plan sponsor may wish to change the method used to allocate profit sharing contributions — maybe from salary proportional to new comparability. This is one of those situations in which the anti-cutback rule described above must be applied on a theoretical basis to determine when a change can occur. The reason is that participants are considered to have earned the right to share in a profit sharing contribution allocated under the existing plan-specified method once they have satisfied all of the plan’s allocation requirements. This is true even when the profit sharing is discretionary and the employer is not required to make any contribution at all.
Consider these two variations on the theme:
A plan that requires participants to be employed on the last day of the plan year to receive a contribution has until December 30th (assuming a calendar year plan) to amend the allocation method.
A plan that requires participants to either be employed on the last day of the year or complete at least 501 hours of service can only change the allocation method up until the date on which the first participant works his/her 501st hour for the year. At that point, changing the method would eliminate a right the participant has already earned.
Plans that do not impose any additional requirements on the profit sharing contribution cannot change the allocation method once the year starts.
See IRS Technical Advice Memorandum 9735001 for additional information.