I find ticket windows to be much faster to get a replacement but guest relations can help too. (unless it's a lost voucher/unactivated) then you probably need to visit guest relations.
I had to go to guest relations a couple of weeks ago when my son AND daughter forgot their magic bands. The GR window was much faster than waiting at the ticket window.
They are both under 18, so I brought my ID as well as my MB (I also had my card on me for discounts).
Took care of it very quickly and both kids were good to go in about 5 minutes.
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