Now... that falsely attributes democrats loyalty to the rules... but I also can't imagine highly placed Clinton loyalists dumping her, short of some catastrophe.
And lets not pretend for a moment that Hillary Clinton would put her Party above herself.
Think Thomas Eagleton
The only one she puts above herself, is her father, Satan.
Rules? Clintons? What?
That said, I don't think they will replace her just because they're afraid she'll lose. They might, however, replace her if they think she's going to drag the whole party down in an embarrassing loss across the board.
For example ... This year was supposed to be an absolute certainty that they'd win control of the U.S. Senate. The Republicans are defending 24 seats, the Democrats only 10. This should have been a no-brainer, but I've seen reports suggesting that the GOP candidate is doing well in almost every state where they were considered vulnerable. The DNC can't possibly just sit back and accept that kind of nightmare scenario.