Your paid subscription starts at the moment of payment and automatically revokes the free days. The free Starter Plan use days will not return if you cancel a paid subscription. Note that the free days are forfeited permanently — they cannot be paused, transferred, or reinstated after a paid plan begins.
You will receive a notification that you have free days left when you decide to upgrade to the paid subscription.
For more details on how free Starter Plan days work, see Can I put the free days of the Starter Plan on hold and use them later? and Are the free Starter Plan days cumulative?.