If the document says the match is discretionary, they do not need to amend it. If they have a separate corporate resolution saying that they will match X% for 2006 that resolution may need to be amended to a new lower percentage, but that would be unusual.
The true up mentioned above would need to be done to make certain that each participant received the same percentage, but that doesn't mean they have to increase everyone to the percentage that was anticipated by those first couple of deposits. So if they matched dollar for dollar in the first part of the year, they may end up with an annual match of 10 cents on the dollar for the whole year, for example.