Once you have exchanged your Gacha Stickers to Gacha Sticker Vouchers, they cannot be restored back to Gacha Stickers.
You can exchange up to 10 Gacha Sticker Vouchers for each Character at the Gacha Sticker Exchange.
Be careful to not trade the Gacha Stickers that you are planning to use to exchange with characters for Gacha Sticker Vouchers.