The traditional view is that from the perspective of Jewish law, a Jew remains a Jew regardless of what they do (i.e. violate Jeiwsh law), although they are "apostates" if they convert to another religion. If they change their minds and wish to return to Judaism, they may be asked to come before a Beit Din (Rabbinicall court) to state their desire for readmission into Judaism and to immerse in a mikveh. These actions are similar to those performed by non-Jews converting to Judaism, but in the case of the apostate Jew, the actions are not conversion, but rather a concrete way to re-affirm re-joining the Jewish people. See for example the first sentence of the Conservative Responsum "The Return of Second Generation Apostates":
http://www.rabbinicalassembly.org/sites/default/files/public/halakhah/teshuvot/19912000/zelizer_apostates.pdf