In order to be able to restore your chat history after changing devices, you need to back it up beforehand.
Before backing up your data, make sure you have:
• LINE version 6.4.0 or later
• iOS 8.1 or later
• iCloud Drive turned ON
Important: Images and stickers sent in chats cannot be backed up or restored. If there are images you don't want to lose, you can save them in an Album or Keep.
To back up your chats:
1. Go to More > Settings > Chats.
2. Tap Chat history backup > Back up now.
for steps to restore your chat history.