While it may be possible to appeal a sentence that is unduly harsh or cruel and unusual, judges are given wide discretion. If the sentence is within the sentencing guidelines, it would be very difficult to show an abuse of this discretion. However, this applies only to cases where someone is sentenced after trial.
In situations like the one you ask about, after plea bargain, the proper remedy would be to make a motion to withdraw your plea. At the time the guilty plea is entered, each judge asks a number of questions to make it difficult if not impossible for you to later change your mind and try to withdraw your plea. These include: "Have you had enough time to discuss this deal with your attorney" "Are you satisfied with the job your attorney has done?" "Do you realize you are giving up your right to trial, to make the DA prove the case against you, to cross-examine witnesses, to call witnesses on your behalf, etc." "Are you pleading guilty of your own free will?" "You are not being threatened or coerced to enter this plea?' "No other promises have been made to you other than the agreed upon plea?" "Are you under the influence of drugs or alcohol?" These questions are designed specifically to negate the valid reasons you might have to get your plea back.
Assuming for argument's sake, that you do have a valid ground to withdraw your plea, there is still more bad news. The judge usually allocutes a defendant after they plead guilty. In other words, the judge makes you swear to tell the truth under oath and admit the facts of the crime you are pleading guilty to. Why is this bad? Because if by some chance you do get your plea back, the case is not over, it merely proceeds to trial and your defenses are hampered/limited by your sworn admissions.
Does this mean the DA can't legally give me a better deal? No. Legally, it would be permissible to alter the original deal once the plea has been withdrawn. But as a matter of policy, a DA would be crazy to give you a better deal than the one you already agreed to. And if he did, the judge would still have to approve the deal which is unlikely. If that were the case, no one would want to stick with their deal and the courts would be flooded with everyone making this motion. The reality is that you are creating more work for an already overworked ADA who believed that a deal was struck. Think about purchasing a new car. You agree to pay a certain price, a lot of work goes into making that agreement happen, contracts are drawn up, financing arranged and then right as you are about to pick up your new car, you call the salesman and say I want to pay $5000 less than I originally agreed. Multiply that by ten and that's how the DA handling your case feels. I wouldn't be surprised if the ADA was so angered by the reneging on the deal that he spends some extra time preparing your case for trial.
In short, don't enter a plea bargain if there is even a slight chance that you may change your mind later on.
All answers are for information purposes only. Answering this question or any future questions does not form any attorney-client relationship. Be mindful, that answers are limited by the limited facts presented by the questioner and are not meant to take the place of competent legal advice by an attorney fully informed of all the facts surrounding your case. However, be aware that nothing posted in a public forum such as this can be deemed confidential or privileged communication. For a privileged private consultation, contact me at 212-385-8600 or via my website www.reasonabledoubtny.com