To everyone, from listeners to advertisers to non-com contributors, a station that changes format becomes a different station. So, in that case, we have a general market station that ceased to operate under their prior format and became a Christian music station.
Just ask an ad buyer at any agency if they are "the same station". In this case, one format ended and another began. You can't say they "lost more than half the audience" as, most likely, the current audience does not at all share listeners with the old format's audience.
And, above that, they are concerned with the message. Donations are a way of sustaining their goal, which involves bringing people to their beliefs.