Can a divorce decree be changed without both parties signing it again?

No but that doesn't mean it won't be changed. Hope he had an attorney

This should not happen, but that is not to say that it can't or won't happen. If he has an attorney, the attorney will make sure that it is done properly. If he does not have an attorney, probably a really good idea to appear at the hearing to ensure that the document that was signed is the same document presented to the court. Even better idea is for him to get representation to ensure that there are no issues. It is much easier (and much cheaper) to ensure it is done right the first time as compared to trying to undo a problem that could have easily been avoided.