If your membership expires and you don't buy a new membership until some time after the expiration date then that will change the monthly renewal date.
I.e. the date on which you eventually buy a new membership becomes your new monthly renewal date.
So you can make the renewal date LATER than it would normally be but you cannot make it EARLIER because your can never terminate a current membership earlier than its normal expiration date.
So if your membership expires on the 26th and you don't buy a new membership until the 4th of the following month then your membership monthly cycle then becomes the 4th of each month.
But you cannot terminate the current membership prematurely BEFORE the 26th and start a new membership earlier than that.
Hope this helps.