Hey @user-removed, welcome to the community!
You can add the year of Premium to your account at any point and it'll continue with Premium as usual.
Also, since you paid for a full year of Premium, regardless of the date you add the new card, the new year will go until the 29th of Apr 2019.
All the best.
“Music acts like a magic key, to which the most tightly closed heart opens.”
― Maria Augusta von Trapp