57:3 Let’s say that one had no particular intention as to whether or not he would eat more fruit, should any more be served to him. Does he then follow the procedure of one who intended to eat more or of one who intended not to eat more? If he has no more of the original fruit left, he must recite a new bracha. (Mishnah Brurah 206:20 disagrees, at the very least if the second fruit is of the same type as the original fruit.) Authorities differ in the event that he has some of his first fruit remaining. It is therefore advisable to avoid this situation by having in mind to include everything when one recites a bracha. If one did not have clear intentions, he should refrain from eating the second fruit in order to avoid the doubtful situation.
57:4 If one was brought a second fruit that was of a more significant type and that he prefers to the first, or if the second fruit is of one of the seven species for which Israel is praised, then he should recite a new bracha even if he still has some of his original fruit remaining. This is because a lesser fruit cannot cover a more significant one without specific intention.