Unfortunately, the likely answer is that the best you can try is to apply for a Governor's pardon. I do not know the facts of your case, but given the passage of time it is most likely that any other options, short of some special deal with the prosecutor, are no longer available to you. You can get the information about pardons off of the State website. Just do a Google search and you will find all the info and the application. you may be better having a lawyer help you with the application process. Good luck.
I agree with the first answer. A pardon is also likely the only option because of the "DUI exception to the Constitution." Like it or not, DUIs are treated differenlty than other offenses. We even have a state statute spelling out the government's intent to vigorously prosecute DUIs and making it difficult to amend a DUI to something else (See Section 967.055: http://nxt.legis.state.wi.us/nxt/gateway.dll?f=templates&fn=default.htm&d=stats&jd=967.055.). So the prosecutor and the judge will likely be very unwilling to amend the DUI to something else, even a misdemeanor DUI. But, you do have compelling circumstances, and you may want to consider hiring an attorney to see if a prosecutor would be willing to agree to reopen the case and amend the DUI to a misdemeanor DUI. In fact, an attorney could investigate all your prior offenses to see if there are any challenges. If one of the priors may be knocked out through a legal challenge, then the OWI-5th would be reduced to a misdemeanor OWI.
Both Mr. Cohen and Ms. Missimer are correct. We have been successful in challenging the underlying record in 5th offense cases, in situations where one of the priors was based on a conviction where the defendant was denied his right to counsel. We have had a 5th offense reduced to a 4th offense while the client was in prison, causing him to be released. This, however, may be dificult, expensive and it will not work in every case. A governor's pardon may be your best bet. Talk to a qualified lawyer. Best of luck. Andrew Mishlove