ALL YOUR MESSAGES WILL BE DELETED. (JK!)
Now we got that out of the way, of course, your precious messages won't be deleted! The Internet is way too advanced and loves to collect way too much data on you these days for that to happen.
Reiterating: No messages will be lost, though there is no easy way to transfer messages from Android to iPhone (or vice versa). Contacts won't be automatically notified or disappear (unless, you choose to notify them or disappear them, and that sounds too freaking ominous for our liking).
If you're connected to a bunch of people in a group WhatsApp, the users in that group will be notified. This happens regardless.
It is highly recommended to go through the official WhatsApp change phone number process to keep everything seamless as possible. Stick in the new SIM, go to WhatsApp More options Settings Account Change number.
After going through the steps, it'll ask which contacts you want notified of the phone number change:
- All contacts
- Contacts I have chats with
Maybe my friends didn't do that, or they just didn't want to keep in touch with me?