Moves bill up rather than speed down after 3 months promotion

Taking a fat change with the billing. I ordered and was on the 20/1 Fibre package from the end of 2019. In December 2020 I moved and took a new line with WebAfrica again and qualified for the free 3 month upgrade to a higher speed. Now on the 1st of april, rather then moving my speed down, they took my bill up to match the faster line over billing me. I either want a refund or have the difference credited to next month NORMAL 20/1Mbps order.