The old wording was:
Cardinals under the age of 80 at the time the Holy See becomes vacant may vote in a conclave.The new wording is:
Cardinals that reach the age of 80 before the Holy See becomes vacant may not vote in a conclave.It may not seem like that big of a change, but there is one important distinction. If the Cardinal turns 80 years old on the same day that the Holy See becomes vacant, he does retain the right to vote in the subsequent conclave.
(Source: #33, Universi Dominici Gregis, by Pope John Paul II, on 22 Feb 1996)