Welcome to Flyertalk and welcome to the BA forum.

There are two sets of policies here. One standard set of policies which allows a change from a cancellation from -3 days to +14 days. And another set of policies for COVID which allowed changes within ticket validity, which would always allow flights to be brought forward, so long as the original ticket was in fact ticketed. So the agent here only referred to the standard cancellation options, and not the other options. Presumably you did have some screen evidence that the flights were rebooked to the earlier dates, and to my mind that makes the booking confirmed from a consumer perspective, even if not confirmed to a ticketing perspective. I would leave it today, and call again tomorrow, ideally you want to talk to a contact centre in the UK or Germany, rather than South Africa or India. These policies are well known and still apply, the current update is version 12, updated on 15 February 2022 and is known as the Covid-19 Principal Customer Guidelines. There are some circumstances where this won't apply, the bit which is problematic is if your original ticket wasn't issued before the cancellation, plus stuff relating to other airlines, travel agents, special interest tickets and so on.

If all else fails, ask for an eVoucher or refund, rebook and then look to recover the difference via CEDR or MCOL. Or get the agent to rebook. However ask the agent to put on the PNR that you are doing this under protest since you believe that the change you are asking for us covered by the Guidelines mentioned above.