Upgrading Mayday

I currently have Mayday Roaside & Recovery but following a bad illness (OH doesn't tow!)  when we were away last month, I decided to upgrade to the Premium Mayday. 'Phoned today to ask if I could update with a pro-rata payment but informed I couldn't because it is too close to my renewal date i.e. 21 Sept. Question is how long before renewal are CC prepared to upgrade such a service. Has anyone had similar experiences?                                       

BTW asked Mayday at the time of my illness and was told quite curtly, "You can't upgrade & that's all there is too it"