If you are on prepaid, a while ago Telecom switched from txt and data plans that roll-over on the first of the month. They were replaced with 'extras' which roll-over every 30 days from the day purchased. Anyway there are two ways to get this done with out calling the contact centre.
1) Call *333. You will have the option to remove txt2500 under option 5(IIRC) and then you can purchase Big Value pack from here as well.
2) from the mobile self service app, Purchase Big Value pack. By doing so a check will be made after BVP has been added and should remove the old plans such as 'Txt 2500 double' within a few hours. As having an old 'plan' and a new 'extra' are a mismatch.
If you are on OneBill or SmartOneBill then at the moment you can only manage the plans/extra (with out calling the contact centre) via calling *333.