What if don't change whatsapp number when changing sim?

Well, yes your friends are correct you can receive the messages even after switching the sim because WhatsApp doesn't depends on the sim card it depends over the data connection. But yo need to make sure that you don't remove the account which is configured over your device.

[Q] What if I want to chat with new friends I'll meet abroad? Do I need to give them my old number to receive whatsapp messages? What if I give them the new number I'll get there?
[Ans]: You need to give your old number to the friends you meet aboard. If you give then new number then I think you can't receiver their messages until or unless you configure the WhatsApp with new number and if you move forward in configuring the new number means the previous account configured is going to get removed and your old friends can't contact you over the old WhatsApp account.

PS: I don't think there is anyway to run two WhatsApp account over a single device parallel ! So, I would say better to get two smartphones or you can continue giving your old phone number to friends for sending messages over the WhatsApp.

Why not just change your number in WhatsApp (here's how) and give your new number to everyone? Then when you move back, put your old sim card back in your phone and change the number back again. It's not ideal, but unfortunately you can't have one WhatsApp account linked to two numbers.

I had seen an app named "2lines for whatsapp" which used to help do the trick but setting it up was very tidious and it used to not function properly. You can check if they have updated it and if it would be useful for you. NEEDS ROOT.

if i change the phone number associated to my whatsapp account as per the instructions you mention, will all my contacts automatically get the new number linked to my account, or will they keep the old number?

i assume that my account will remain completely unchanged (contacts, conversations, etc...) after the procedure?

cheers

You can get a Google number, and that can work on your phone even when you change your number on your phone. Give all of your contacts your Google number, and they can call both that number and your new number.

Yes you can use the same number, but in case if you have to reset the your phone, you will lost that number whatsapp, so it is better to use new number as a whatsapp number.

On that note.. Will I need my old sim to receive a confirmation text abroad? Or will a message pop up when going on whatsapp with new sim asking if u want to keep old number or switch to new? So basically will I need my old sim number active abroad to keep my old number?