I wonder if the agent has misunderstood what needs to be done. OVO has an established procedure for those in your position: simply to set the peak and offpeak unit rates to the same figure. The meter carries on as before, recording usage separately during peak and offpeak periods. I know it works - it was done for me a couple of years ago with no problem at all.
The agent seems to have started the process to change the national database record for your meter from multi-rate to single-rate, which isn’t really necessary or appropriate: the meter is still a multi-rate one, even though the rates are the same. Changes to the database happen at a glacial pace, which is why you’ve been warned about a possible delay of 35 days before it’s complete.
Perhaps @Emmanuelle_OVO could check that what she wrote in December is still valid and find out whether the agent has done the right thing.