It probably won't hurt.
Hillary's moved to the left from Bill Clinton's position as Governor of Arkansas, so it would be tough for her to make an ideological purity argument.
Warren's convictions seem genuine, so it isn't a matter of flip-flopping.
It could help Warren with electability arguments. Who better to convert swing voters than someone who voted for Republicans?
If it didn't work for Crist and it didn't work for Specter, it's probably not going to work for Warren.
Both parties, the voters see party switching as literally crossing over from the allies into the nazi party.