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The Rule 11 agreement would stay in place, even after your lawyer has withdrawn. Each party has to sign the Rule 11 agreement, so it would stay intact regardless of whether you were represented by a lawyer.
That said, a Rule 11 agreement can be modified if it is no longer practical for you since you are pro se now. You would have to file a motion to modify it though and get the judge's approval.
Also, keep in mind, that a pro se litigant is not given any special treatment, you are treated the same as a lawyer, so you would have to go about the process of modification in the same way.