I don't think you can actually buy vouchers at the ticket booth; I always thought they only sold the Annual Passes themselves there.
In terms of upgrading tickets...yes, but it has to be a DLR ticket. They used to be able to deal with WDW tickets, but most of the recent reports indicate that the DLR ticketing system can no longer talk with the WDW ticketing system, so they can not do anything with WDW tickets at DLR.