While it is possible to get FEC approval to change your mind, since the FEC is short 3 members (due to Congress stonewalling the Bush admin.) the FEC does not have a quorum (during a national election for President and Congress). Without a quorum, the FEC cannot approve McCain's flip-flop, so he may be acting illegally.
But what the heck, he probably will only get a $100,000 wrist-slap after the election, a la Billary's many fundraising fiascos.