Penal Code 1203.4 provides for the withdrawal of the conviction and dismissal of the original charge. Therefore, as a matter of law, you have never been convicted of the DUI.
Since the question was whether or not you have ever been convicted of a serious misdemeanor, then presuming this is your only offense, the answer of "no" is the truth.
As for what makes a serious misdemeanor, there is no statutory definition. The general rule is that where the consequences of a misdemeanor is substanital enough to require that an indigent defendant would be permitted appointed counsel at public expense, then such a misdemeanor is serious. See, e.g. People v. Wong, 93 Cal. App. 3d 151, 155 Cal. Rptr. 453 (Cal.App.Dist.2 05/17/1979).
Hope this helps.
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