Thibault switched pleas because, according to her lawyer, she came to a better understanding of the evidence and the law.
It was Thibault's first appearance in court since August when she suffered an epileptic seizure that interrupted her cross-examination.
The charges were laid after a joint report filed by former provincial auditor general Renaud Lachance and his federal counterpart at the time, Sheila Fraser.
The document suggested more than $700,000 in alleged improper expenses had been claimed.
Her trial heard the money was allegedly spent on gifts, trips, parties, meals and skiing and golf lessons.
Thibault held the provincial vice-regal post between 1997 and 2007.
Quebec court Judge Carol St-Cyr will hear sentencing arguments as well as the results of negotiations on a reimbursement plan next May 1.
Thibault, 75, left the courthouse on Monday without responding to reporters' questions.
Her lawyer, Marc Labelle, said the former lieutenant-governor came to a new understanding of the evidence and the law.
"If you think you didn't commit a crime, but then you see through the interventions in court, you see how things are presented, the questions from your own lawyer, at a certain point you are able to say that, maybe there are things I did that were not acceptable, legally speaking," he explained to reporters.
Labelle said his client acknowledged she unjustly claimed an amount of $310,000, while the Crown estimates the amount at $430,000.
"We will discuss . . .to see if we can meet somewhere between $310,000 and $430,000 and we will then make a representation to the court," he said.
But Labelle would not say if his client had the financial means to make the reimbursement.
During her testimony, Thibault said she had little to show financially for her time as vice-regal — a divorce ate into her savings and she now lives on a $30,000 pension.
Her lawyer also did not indicate Monday what sentence he wanted to negotiate with the Crown in order to avoid Thibault being imprisoned.
"We will discuss that with the judge," he said. "I don't want to start talking about a sentence in the corridor.
"Regardless of what I think about jurisprudence, it's Judge St-Cyr who will have the last word."
Last summer, St-Cyr ruled against a pair of motions filed by Labelle, who argued the case should be dismissed because the accused benefited from royal immunity.
He contended that meant she was not a civil servant and therefore could not face criminal charges.
The judge wrote that, according to constitutional law, the lieutenant-governor does not enjoy the same benefits as the Queen.
St-Cyr also noted that under the Constitution, the lieutenant-governor is a civil servant, adding such an affirmation is even posted on the lieutenant-governor's website.
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