Beware when you downgrade

On 28/12/23, I downgraded from Mega Talk L to Mega Talk M. My Dece3mber invoice already must have been processed with the Mega Talk L charge for the period 27/12/23 to 26/1/24. On receipt of the January invoice, i was invoiced 2x for Mega Talk M - once for 27/12/23 to 26/1/24 and again for 27/1/24 to 26/2/24. I cannot get anyone at MTN to understand that I cannot possibly use a Mega Talk L and Mega Talk M at the same time for the period 27/12/23 to 26/1/24