That is a question for your lawyer. You may have to use all the money for the plan anyway, and now the Ch. 7 Trustee has a claim in some amount that might have to be paid in full. We don't know what your plan would be or anything about your case. Having made a big mistake already, it is more important now to have your lawyer explain the options.
Conversion from 7 to 13 requires permission of the court. The trustee will object, arguing that he will lose fees on the found money, and that you committed perjury by failing to disclose the bank account. If the court allows conversion, it will likely allow the trustee to file a claim for the lost fees and expenses associated with finding the money. If your conversion is allowed, you would have to file a motion with the court to get a dismissal, and the court could just put you back in the 7 instead. Perhaps it is time to hire an attorney to try to extricate you from this problem - expect to pay more than if you had an attorney from the start, as undoing problems are often much more time consuming - and there is no guarantee of success.
The easy part wold be converting to a Chapter 13. The harder part is successfully completing a Chapter 13 so as to get a discharge. It may be better off for you to let the Chapter 7 trustee liquidate the cash which was found, in exchange for a discharge. Withdrawing a 13 before discharge gets you nowhere, as you are back to square one with your creditors, with no fresh start to protect you. Certainly something to review with lawyer who has been through all this before.
I am not your attorney unless you and I have signed a retainer agreement. What I am saying is not legal advice. Do not act on this information without engaging my services, this is for consideration only.
The first thing you should do is discuss your available exemptions with a good experienced ch 7 attorney in case there are exemptions available to you that you are not aware of , then file amended Schedules B and C if there are. After 30 days after filing any such exemption, if the trustee has not objected then you are fine. The trustee only receives $60 in a no asset case which is 99% of all cases, so they don't normally waste time on a case where the unsecured creditors would not end up with much in payout of non exempt assets. If you convert, you do not have the automatic right to dismiss your ch 13 and a trustee or court could object if the result is the funds that are not exempt would be lost in that they would not be used to pay your creditors and thus force your case back into a ch 7 possibly! Good luck.
You can convert, but that might be even more expensive. You can ask permission to withdraw, but it is up to the judge to grant or deny that permission. The Trustee can object if he/she thinks you are trying to pull a fast one.
You can ask the court to convert your case, but you will have a hard time getting out of bankruptcy all together, at least in my district. If you file a 7, you can withdraw it easily. If you file 13, it will be dismissed if you miss your payments. If you convert from 7 to 13 and then try to withdraw or stop making payments, the court is likely to put you back into a 7.
This is not legal advice and I am not your attorney until you retain my office. Always consult with an attorney in your area before acting on anything you read on the internet.