All your data and purchase history is linked to your Between account.
If you connect with a new partner using your existing Between account, you can continue using all your previously purchased stickers.
If you can't find the stickers you purchased, go to the More tab > Purchase History > Restore Purchase.
※Caution
1. Please be aware that your purchase history also gets deleted when you delete your Between account.
2. Purchased stickers cannot be transferred to different IDs.