The ZED tickets are mileage based if it is the same zone it should be accepted. In some cases the tax need to be recalculated and it can become an agent decision.
If let's say you have a ticket for a zone 9 and want to use it for zone 8 it should be accepted but you will not get any refund.
The airpiort agent can be contractors and not real airline employees in some airport they might not know the "subtility" of ZED tickets.
I always carry a print out of the rules to show if needed.