Yeah. A lot of people said that Miguel Cabrera would hit 30 HR this year. I have heard some people say that because he has already hit around 10, then he should only hit about 20 the rest of the year. BS of the highest form. Because of his hot start, I would project him now to hit around 40 HR, maybe a little less. That means that he would hit what people projected for him before the season the rest of this year.
MCab is not the best example, cause he is totally unproven and we dont have any numbers on him in the bigs over a full season at all, so we cant project for him based on past experiences. Jeter is a much better example. If you are trading for him, expect him to hit around 300 the rest of the year, because that is how he has hit his entire career. He will have some cold and hot streaks, but he will not all the sudden go crazy and hit 500 to bring his overall average up to par with the rest of his seasons.
Basically, dont stick with your pre-season projections for this entire year; take those projections and shrink them down to fit the rest of the season, and you have how that player will do for the rest of the season. However, if the player is either young or at a changing point in his career (after an uncharacteristicly bad year, an injury, etc), dont think that they will necesarily return to that bad form, especially if they are playing well right now.