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DearCustomer- You would have to file a motion to withdraw your plea. You could use the fact that you were not represented by counsel and did not understand your rights. I doubt the court would agree and you would then be forced to file an appeal with the court of appeals. all of this will cost quite a bit if you do get a lawyer and without a lawyer it will be virtually impossible to proceed.
A six month suspension is not a severe sentence for driving under suspension and if you are successful in changing your plea you run the risk of an even greater sentence and possible jail time since the court would not be bound by the original sentence of a six month suspension. I would be very careful trying to do this but you certainly have the right to try.
You can plead not guilty in the other two cases and request a court appointed attorney. Your original question was regarding changing a plea but if you haven't resolved the other case or cases you certainly can contest those.
Most courts will grant privileges for work or school so you should certainly apply. You indicated you had cleared up your previous problems so you should be able to get privileges but it is ultimately up to the court.