How can I merge my friend's new phone number in WhatsApp without losing all the chats history (merge chats from two different numbers to one number)

I've been using WhatsApp with my best friend for years and he swapped his prepaid simcard (old phone with with number = X) with a new prepaid simcard (new phone with number Y) recently. When he sent me the first message using the new phone with number Y, I received a new chat window because his number Y was not registered on my phone contact's list. When getting this new chat, I blocked the number because it was unknown to me. When he gave his new number Y, I added it to his contact record on my phone but chats didn't merge together and this is not what I had expected, because now I have our two separate chats: the old one and the new one. How can I merge the 2 chat windows together and keep only the one with the whole chat/calls/medias history? Thank you, Bruno Summary : Is there any way to Merge two different WhatsApp chats into one single chat ? Practical issue being someone used two different numbers to text.