A lot depends on what stage you are at - Series A/B/C? Do you have VCs or angels? But the worst thing you can do is to come across as unreliable or a cheat, so

A lot depends on what stage you are at - Series A/B/C? Do you have VCs or angels? But the worst thing you can do is to come across as unreliable or a cheat, so if you are having doubts - talk them over with your investors. After all they are your partners, and are equally interested in seeing your company succeed. If you screw them over chances are you will be banned from the VC community for good, which ruins any chances a new startup would have. Talk the idea through with them, they will have insights that will prove useful. If they get equally excited you will find a way out together that makes all parties happy. If they don’t - perhaps the opportunity isn’t as great as you think, otherwise they’d want to be a part of it too. Treat them like you would want to be treated yourself.