No, they cannot be changed. When you make a new island, you’ll have to keep in mind the locations of Resident Services, airport, North beach (where Redd visits), river mouths, dock (where Kapp’n is), and extended land peninsula. These are all things you cannot change later, so make sure you’re somewhat satisfied with what you pick.
If you decide to make a new island and are not satisfied with the maps that were shown to you by Timmy & Tommy, you can close the game at that point to start over and get a new map selection. Keep in mind that it’s sort of a tedious process to do over and over, so be prepared to have a bit of patience if you’ll do this.