I found an Eastern article that seems to indicate that chrismation is not necessarily a one time only kind of thing like baptism is, but in extreme circumstances (such as a returning apostate) could be administered again. (It's under the heading "Dealing with other Christians").

Regarding this, something else you might find interesting is Canon 95 of the Quinisext Council, which might give some clarity to how we "Deal with Other Christians."

THOSE who from the heretics come over to orthodoxy, and to the number of those who should be saved, we receive according to the following order and custom. Arians, Macedonians, Novatians, who call themselves Cathari, Aristeri, and Testareskaidecatitae, or Tetraditae, and Apollinarians, we receive on their presentation of certificates and on their anathematizing every heresy which does not hold as does the holy Apostolic Church of God: then first of all we anoint them with the holy chrism on their foreheads, eyes, nostrils, mouth and ears; and as we seal them we say--"The seal of the gift of the Holy Ghost."

But concerning the Paulianists it has been determined by the Catholic Church that they shall by all means be rebaptized. The Eunomeans also, who baptize with one immersion; and the Montanists, who here are called Phrygians; and the Sabellians, who consider the Son to be the same as the Father, and are guilty in certain other grave matters, and all the other heresies--for there are many heretics here, especially those who come from the region of the Galatians--all of their number who are desirous of coming to the Orthodox faith, we receive as Gentiles. And on the first day we make them Christians, on the second Catechumens, then on the third day we exorcise them, at the same time also breathing thrice upon their faces and cars; and thus we initiate them, and we make them spend time in church and hear the Scriptures; and then we baptize them.

And the Manichaeans, and Valentinians and Marcionites and all of similar heresies must give certificates and anathematize each his own heresy, and also Nestorius, Eutyches, Dioscorus, Severus, and the other chiefs of such heresies, and those who think with them, and all the aforesaid heresies; and so they become partakers of the holy Communion.