All five major schools of Islamic jurisprudence agree that a sane male apostate must be executed. A female apostate may be put to death, according to some schools, or imprisoned, according to others.
Some contemporary Shi’a jurists, scholars, writers and Islamic sects have argued or issued fatwas that either the changing of religion is not punishable or is only punishable under restricted circumstances, but these minority opinions have not found broad acceptance among Islamic scholars.
A specific legal framework is essential to most variants of Islam, so there are explicit precedents for how an apostate should be treated. For example, there are differences between the schools in how long an individual can be allowed a chance to recant their apostasy so as to save their life (there is often a difference between men and women as women are given more latitude and time to change their minds). Since Islam arose and maintained a religiously pluralistic environment in the Middle East (there were large Christian minorities) the ideas surrounding apostasy are relatively widely developed. This is clearly a case where Islam needs to be updated from its medieval stasis. Christianity also had its own phase of death penalty for apostates, there were harsh penalties for converting to Judaism (there are instances of Judaizers immigrating to Muslim Spain from France), and of course heretics from “orthodox” Christianity (e.g., Cathars) could be subject to capital punishment.
Finally, the Mufti did say:
Gomaa warned however that if the conversions undermine the “foundations of society” then it must be dealt with by the judicial system, without elaborating.
This is interesting, and shows the legalistic bent of Islamic scholars. Other societies do have concerns about conversion. Russia has legislation which gives clear most favored status to “traditional” religions and curtails unorthodox sects. Indian states have often attempted to pass legislation restricting conversions. The government of Singapore has expressed some concern with the proselytizing activities of some Christian fundamentalists. Islam on the other hand transforms this normal and conventional sentiment (i.e., societies which preserve their general values and beliefs into the future) into a systematic framework. This explains why one of the arguments for capital punishment in relation to apostasy was that it was “treason” against the Muslim nation. Ultimately these sorts of rulings will, hopefully, lead to the eventual withdrawal of Islamic legalism to the private realm so that Muslim nations can become countries where Islam is the predominant religion, not where Islam is coterminous with national identity.