As a few have said, you can go to the Ministry in person and get a valid sticker anytime, even after the old one expires.
However, this is my understanding.
- If you keep your same plates, and renew sticker after the old one expires, you still back pay the previous months. That is, you pay for the full year (even if it's 6months expired). The new sticker always expires on your birthday. That's what happend to me for my stored cars.
- If you change your plates, you only have to pay for the remaining months. I've done this before, as the cost of renewing the plate was more than getting a new plate and remaining months of valid sticker.