So what will happen with clients current bonuses? If the company wants clients to use their app instead of loyalty cards, will clients accumulated bonuses disappear?
Nope

All loyalty points of the companies that will employ our app will be added to blockchain.
dev please explain this from the technological point of view
Blockchain stores clients hashed phone number. After client got registered and authorized in the WULET system, their bonus points from all WULET partner companies they are registered in will be automatically accrued to their personal balance. It happens due to the hash match. When a new partner company joins the WULET system, its clients will be automatically accrued the bonuses theyve accumulated by participating in its loyalty programmes.