Rhetorical: The implicit is made explicit. Alma the Younger specifically asks the current congregation if they have undergone that "mighty change." In the context of the contentions of the times, one of the implicit meanings of the "mighty change" is to give up the temptations of the ideas of the world such as the Nehors, and to stick to the word of God. Alma the Elder's "mighty change" was precisely such a renunciation of the competing religious ideals and a full acceptance of the word of God. Alma the Younger is asking his people if they too have made that "mighty change," a shift away from the temptations of the world and toward the word of God. This current population should expunge the feelings of desire for the things of the world (typified by the Order of the Nehors) and place their faith and efforts in the word of God.
The spiritual birth refers to Benjamin's declaration of the nature of the covenant his people made:
Mosiah 5:7
7 And now, because of the covenant which ye have made ye shall be called the children of Christ, his sons, and his daughters; for behold, this day he hath spiritually begotten you; for ye say that your hearts are changed through faith on his name; therefore, ye are born of him and have become his sons and his daughters.
Alma's "spiritually born" should be understood in this context. He is speaking to the members of the church, and clearly this commitment to becoming born of Christ, to become his spiritual children, continued to be part of the church, even if it were no longer a covenant accepted by all of the people Zarahemla. That this particular spiritual rebirth should be seen as part of the covenant with the Christ will be further developed in the next verse.
Spiritual: Of course we do not need the specific social context of Alma's discourse to feel the import of these words in our own lives. For all of us, the effect of a complete acceptance of the gospel is to create a change in our hearts. We too have a competing worlds around us, and we too need to mightily change our hearts away from those temptations that surround us. Just as the ideas of the world were for Alma the Elder's people the "chains of hell," so too can the world today wrap us in chains of bondage.