About changing phone numbers -
The change number feature allows you to change the phone number associated with your WhatsApp account. Changing your phone number in WhatsApp will:

Transfer your account information, including your profile photo, name, about, individual chats, group chats, and settings from your old phone number to your new phone number.
Delete the WhatsApp account associated with your old phone number, so your contacts will no longer see your old phone number in their WhatsApp contacts lists.

You can choose whether you want your contacts to be notified about your phone number change. Your group chats will be notified when you change your phone number regardless of whether you choose to notify your contacts.