Most likely, you won’t be able to keep a larger bedroom size unit. But if you have a member of your family with a disability, and that person needs the extra bedroom to store medical equipment or cannot share a bedroom, or needs the extra space for another reason connected to the disability, you may request a reasonable accommodation to keep the original bedroom size voucher.
As always, you will have to supply the required proof of the need and the request must be approved by the PHA. Also, if the rent is low enough, and your income is high enough, then you may keep the original oversized unit or find another unit with very low rent. In that case, the actual bedroom size of your voucher will still be lowered to the correct size and your portion of the rent might substantially increase.