I had one reissued just a week ago (before leaving). The travelagent was unsure about the change in Airport taxes / surharges / whatever. So I thought I quickly visist the BA counters at the airport in CPT. The ticket was issued on QF stock (QF does not have an office in CPT), Not only were they most helpful and friendly, they reissued from paper ticket to e-ticket within 5 minutes and did not even charge for the additional taxes which must have been due as three segments were added. They only charged me the obligatory fee of US$ 75.00 for a rerouting.

I would visit any O/W office which is convenient to you.