Not sure what you mean.
If you buy a new membership now it will not start immediately.
It will stack and kick in at midnight on the 13th.
Until then you won't be able to use the new membership bundled allowances.
If your problem is that you have used up your membership bundled allowances on your current membership then you need to buy add-ons to tide you over until the 13th.
All memberships always run their full monthly course and there's no way to prematurely terminate one and have a new one start immediately.
I don't really understand your points about "extending" membership or why you mention getting a new SIM!?
Hope this helps.