Innocent people are coerced into pleading guilty all the time, and they can withdraw it.
1. It's generally easiest to do BEFORE the judge signs off on it. That wouldn't apply here.
2. It must be done for a good reason, and should be done soon after it was signed. It's almost unheard-of for a judge to allow a guilty plea to be withdrawn more than a year and a half after it was entered.