This is an interesting quote because we often use the words change and modify as synonyms of each other. However, it is important to note that even though we have two words that means something so similar, there must be a distinction between them.
Change makes something completely different. Modifying makes something altered. I would look at the two of these as if modifying is a small degree of change.
Therefore, in Lord of the Flies we see the boys become modified because they move from irresponsible children dependent on adults to partially being able to meet their own needs. For example, they seek food, shelter and safety on their own; but they do get hungry because they can't get a pig, they fear the beast, and their shelters are built by only a few of the boys and the shelters are capable of easily being destroyed. You can give these children adult responsibilities, but that doesn't make them adults. Therefore, they tried to change, but they cannot completely change. They modified.
Another example is the fact that people will always trampled each other for power. Jack was used to having power as the lead choir boy. When Ralph was elected leader, Jack was hurt. Jack did his best to overcome this by accepting the role of leader of the hunters, but it was not good enough for him. Jack tried to change his human nature, but he could not. He accepted the role for a time (modification), but he could not take it for the entirety of the book and this is why he breaks off from Ralph's leadership establishing a tribe of his own. He could not be changed. This tribe resulted in further destruction of others which is very much a part of Jack's nature.
When the boys have a meeting in chapter 4 because the order of the group is disrupted, they demonstrate modification, but not change. Children are disobedient. When they tried to have a society with roles and rules, it worked for a time and with some success (modification), but ultimately the children remained disobedient to the newly established rules (like where to use the restroom so food supplies aren't ruined).