Extending or renewing an existing subscription while it is not yet expired will not make it lose the remaining days you still have with your current subscription. Renewing BEFORE expiry date will just extend your subscription to a NEW EXPIRATION DATE.
To understand better, here is a sample:
You have a current subscription of Classic package expiring on September 30, 2018, 12:00 p.m, but you want to renew to it for another 3 months on September 28, 2018, 2 days before the expiration date.
You will not lose the 2 days remaining on your current subscription, rather it will just extend the expiration date to December 30, 2018, 12:00 p.m.