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Googled your question:

Pre-Sentence: It Might Not Be Too Late

A defendant can withdraw a guilty plea that a judge hasn’t yet accepted. Also, defendants who have pleaded but not yet been sentenced can sometimes get out of their deals, particularly when the judge rejects the negotiated agreement pursuant to which the defendant pleaded. For example, if Clay pleads guilty to bribery in exchange for the prosecution’s agreement to a three-years-or-less sentence, but the judge indicates an intention to sentence him to five years, he can probably withdraw the plea.
Post-Sentence: A Tall Task

Some plea deals require the defendant to waive the right to appeal; not surprisingly, undoing a plea in these instances is particularly tough. But even without a waiver, once a judge has sentenced a defendant, the odds of getting out of a plea can be long.
After sentencing, the trial judge will typically set aside a conviction and allow plea withdrawal only if it’s necessary to avoid an obvious injustice. It’s not enough, say, that the prosecution agreed to and did recommend a certain sentence as part of a plea deal, but the judge imposed a longer one. After all, the judicial system prioritizes efficiency, and rehashing cases is no way to speed the docket along. In addition, judges entertaining plea withdrawals are supposed to consider their prospective effect on the prosecution. If, for instance, the prosecution lost contact with witnesses who were necessary for trial between the time of the plea and the attempted plea withdrawal, the judge might deny the defendant’s request.

https://www.nolo.com/legal-encyclopedia/withdrawing-guilty-plea-criminal-case.html

In this case he “pleaded guilty” (assume that means the judge accepted the plea?) but obviously has not yet been sentenced. I think that if Mueller knew that the biased agent reassignments story was about to break (he certainly knew that Congress had requested the info) prior to the guilty plea (and assuming Flynn did not know), then arguably the guilty plea was obtained by fraud. I would love to see the general move to withdraw the plea.

What he can\can't do is generally specified in the agreement.
Here's that part of Flynn's agreement:

https://www.justice.gov/file/1015121/download Nothing in this Agreement limits the right of the Government to seek denial of the adjustment for acceptance of responsibility, pursuant to U.S.S.G. § 3El. l, and/or imposition of an adjustment for obstruction of justice, pursuant to U.S.S.G. § 3C 1.1, regardless of any agreement set forth above, should your client move to withdraw your client's guilty plea after it is entered, or should it be determined by the Government that your client has either (a) engaged in conduct, unknown to the Government at the time of the signing of this Agreement, that constitutes obstruction of justice, or (b) engaged in additional criminal conduct after signing this Agreement.

and... Your client acknowledges discussing with you Rule l l(f) of the Federal Rules of Criminal Procedure and Rule 410 of the Federal Rules of Evidence, which ordinarily limit the admissibility of statements made by a defendant in the course of plea discussions or plea proceedings if a guilty plea is later withdrawn. Your client knowingly and voluntarily waives the rights that arise under these rules in the event your client withdraws your client's guilty plea or withdraws from this Agreement after signing it.

any breach of this agreement does not give Flynn the right to withdraw

This bleeding agreement basically lets any LEO from any jurisdiction beat Flynn like a rented mule.