Based on the information you have provided, I do not know of a legal basis to go back and change your sentence. If the judge accepted your negotiated plea and simply misstated the adjudication issue, then you can go back and ask that his mistake be corrected. In order to do so, you'll need to pull the transcript from your plea hearing and any related documents that would show that you were to receive a withhold. If the judge refused to accept your plea agreement because he/she wanted you to be adjudicated guilty, you're simply out of luck, barring a Governor's Pardon.
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I agree with Mr. Ladan and would add one thing. Many times for a withhold of ajudication, the judge will insist on a guilty plea. Except for a few civil matters, pleading "guilty" versus "no contest" is of no consequence to your criminal case. The important issue is if you received a withhold of ajudication or a conviction (ajudication).
I would also agree with Mr. Ladan. I would add that if you truly did not understand you were being adjudicated, then you could have possibly set this aside especially right after the plea. Since this was over five years ago, I would imagine it will be difficult if not impossible to set aside your conviction.
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You may have a viable post-conviction claim.
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