Why wouldn't the DNC change candidates IF the "One" is faltering in the polls, etc?

Because the DNC is determine to nominate Obama in order to keep the black community within the party. Obama's nomination is not for the purpose of winning the White House in November, but to give the black community and extreme liberal democrats a reason to stay with the party in the long run. The DNC's goal is to sacrifice the election in order to keep the party base in line for the long term. Now they can lay claim to the black vote for decades. So if Obama were caught on tape beating a 5 year old white girl, the DNC would still not change to another nominee.