How are you seeing A available? EK usw mareied segment Logic, so Just because XXX-DXB and DXB-YYY have A available individually doesn't mean that there is A for XXX-YYY.

Generally the TA should be able to open the PNR, remove and add segments and then reprice it, but the change in departure city means the fare basis is probably different which is why this isn't working.

Ask the TA to cancel and rebook, but hold the credit from the cancellation, not refund it, and only bill you the difference+cancellation fee