According to the letter they already switched course and it didn't work out.
Some term sheets may not allow you to drastically switch course. I'm guessing this is more likely the larger the round.
Either way, the right thing to do is to give the investors the option to cash out.
http://www.paulgraham.com/die.html
AppJet did a good job following that advice IMO, but Heroku takes the cake. They didn't just change direction. They kept the old product alive, too.
Edit: I was wrong. AppJet kept some of their old stuff around, too! I can't wait to check it out. http://appjet.com/dev-archive.html