It probably should be obvious, but this refers to changing the formal definition of an ampere, but not to changing its value in an practical terms. An ampere of current will be the same amount (at least within several decimal places) with either definition. The advantage is that the new definition will be easier--indeed, in this case, possible--to replicate from scratch.
Similar redefinitions have been made to other base units as well. At various times the meter, for instance, has been defined in terms of a fraction of the circumference of the earth, as the length of a certain metal rod kept near Paris, as the number of wavelengths of a color of light, and as the distance light travels in a certain fraction of time. The actual distance a meter represents has not changed, though each successive definition has become more precisely repeatable and testable.
