METASCORES can change, and in fact can do so frequently. The main culprit behind these changing scores is the addition of new reviews.
In a perfect world, all of our publications would have a review for the movie, game, television show or album prior to its release. However, in our world, this is not the case, and reviews trickle in over a period of time. Thus as we continue to add reviews over time for a particular products, its METASCORE can fluctuate (sometimes many times within a day, as can be the case with new movies and games). Because the METASCORE is basically an average, it will fluctuate more when a new review is added if there are only a few reviews to begin with, and won't change too much if there already are many reviews.
For major-release movies and television shows, scores typically will not change much over time, as most reviews will be published on or before the day of release. (We will usually get all of those reviews posted by late morning of that day.) For a limited release film, reviews trickle in over time as the film is added to new geographic markets. For games and albums, reviews are even less standard and will show up anywhere from one month before to three to six months after the release date.
Note that the number of reviews is listed at the top of every page, so you can tell at a glance when more reviews have been added. (Shameless Plug:) So to make sure you have the most current METASCORES, be sure to check metacritic.com often!