If you haven't opened your Stamp Packs or Pet Boxes from the last season, here's what happens:
1. Pet Boxes will be converted into coins.

2. Unopened Stamp Packs will remain in your inventory, and you can still open stamps from the last season. However, you won't be able to open stamps from the new season until you get the new packs.

Be sure to open your items before the season changes to make the most of them!