Yes you can, activating your voucher simply provides booking information for your current experience, this does not effect your expiry date or whether you can exchange the voucher. Providing you have not made a booking and your voucher is in date, you should be able to exchange your voucher.
To exchange your voucher, simply visit My Voucher and it should offer you a button that says "Exchange to a different experience".
*Vouchers purchased corporately or obtained through a competition win cannot be exchanged. Event tickets are also exempt from exchanging.