Am I bound by the prenuptial agreement if I change my mind later?

In all likelihood, yes, you will be bound by a pre nuptial agreement. A prenuptial agreement is an enforceable contract. Unless there is some very good reason for a judge to let you reject or modify the deal that you made, you are likely to be bound by it even if you later decide that it was a bad deal.

If, however, your circumstances change dramatically in a way that could not have reasonably been anticipated - such as a serious illness - the judge might require your spouse to help you despite the premarital agreement. That will depend upon the law of your state and on how drastic the change of circumstances is and how unlikely it was that your situation would change. For example, it is much less likely that unexpected health problems would arise in a 30 year-old than in a 78 year-old. For more information, consult with a family law attorney in your area.