Among evangelical Christians, the term "apostate" is usually reserved for someone who professes Christianity for a time, then turns completely away from it (and generally, opposes it in some way). I've heard Bart Ehrman referred to as an apostate, because he professed Christianity for many years, graduated from Moody Bible Institute (graduation from Moody requires that you affirm your faith in a statement of basic Christian beliefs), went to Princeton Seminary and worked under Bruce Metzger in textual criticism, but now refers to himself as an agnostic and publishes books that attempt to debunk the accuracy of the Bible. A Baptist who becomes a Methodist would not be considered an apostate in most circles (and the few churches that might would likely be thought of as near-cultic), nor would someone who just happens to disagree with certain tenets of the faith but still professed belief in Christ. The application of the term normally requires a departure from Christianity in any form.
Last year, I left a Baptist church to join a Christian & Missionary Alliance church, but I am still very welcome at the Baptist church any time I visit, and have good friends there who would never think of shunning me. They do not regard me as having somehow departed from correct teaching, and, in fact, I was teaching their adult Sunday School classes right up till the week I left (and, I suspect, would be quickly tapped to do so again if I were to return).