Previously stolen sim reactivated, what will happen when I change number back?

Hi,
I'm a little confused about WhatsApp numbers and devices and what I should do in this situation. I'll try keep this brief!

I had my phone stolen abroad while travelling with my sim in.
I bought a new phone and sim. I couldn't transfer my WhatsApp account to the new number (let's call number B) as you need the old sim card (number A) so I made a totally new WhatsApp account I used for the rest of my travels with num B.
I am now back home and have managed to get a new sim with my old number (num A) from my provider.

I just put that in my new phone but if friends send to num B. I still get the messages, and I got no backdated message from num A account when I put the new sim in the phone.

Soooo. My question is, do I just need to use the change number feature on the new phone from num B to A, and I'll get all the backdated messages from my old number?

And there is no way to 'merge' the two accounts right...?

Cheers!
Dan

I see, thank you. Do you think all the messages that were sent to number A while it was lost will come through when I change the number over?

Also, I should be able to change back and forth to the old and new numbers by changing it in WhatsApp now and again to check for messages?

Cheers!