How to backup viber from iphone 6s to new iphone 14 with new icloud

Hi,

I have specific problem and can't find solution for it.

I have old iphone 6s and want to transfer viber chats to new iphone with new icloud.

Old iphone 6s: I can backup viber but just with icloud. (I dont have "Back up now" button as on new iphones)

New iphone (new icloud): I can not restore backup from other icloud.

So, any solution how to transfer viber?

Maybe: What would happen if I sign out from Apple id on my old iphone, and sign in with my new Apple id? Will viber chats stay the same or it will be deleted?

I think one of these 2 options will happen:

1. Viber will be backed up on my new icloud on old iphone?
2. New icloud will delete viber chats when I sign in on old iphone?

Thank you