Marriage, Remarriage, and Excommunication

The full link is here to Canon Law Made Easy. The original question was posed about a Catholic woman who remarried outside the Church and became an Episcopalian because she said she was excommunicated.

 First of all, let's distinguish between between an act that is morally wrong - sinful, in other words - and one that constitutes a crime under canon law... there are many acts that are sinful, but they are not punishable under canon law. Lying, thieving and adultery are all objectively evil actions, and moral theology teaches us they are all sins if performed by someone who understands that they are wrong, and yet freely does them anyway. . . But lying, thieving and adultery are not punishable under canon law as crimes.

This is why...divorced and remarried Catholics (whose first marriage was never annulled) are not able to receive the Eucharist as long as they persist in living with the second spouse as man and wife...

While, the current Code of Canon Law. promulgated in 1983, contains no sanction against divorced and remarried Catholics, the 1917 Code of Canon Law did. The former canon law 2356 addressed the crime of bigamy.

Bigamists, that is, those who attempt another marriage - even if only a so-called civil marriage - while the first conjugal bond exits, are ipso facto notorious; and if the scorn the warning of their Ordinary and persist in this illicit cohabitation, they are excommunicated...

Note from the wording that that there was nothing automatic about this: if the Ordinary never found out about a Catholic's remarriage, and/or never issued a warning, there couldn't have been an excommunication.

If this was the law under the old code, why was it changed? The super short answer to that question is that Pope John Paul II clearly didn't clear to include it in the new Code of Canon Law, which he promulgated himself.