Upgrading phone but staying on same network

[Simon_16v]

Bit of a hard one to give an accurate topic name...

My Mrs is currently on Three and her contract is about due to end. I've found a decent deal on an iPhone 13 through buymobile.net, however it is also on Three.

What is the easiest way to take the deal, get a Lebara sim for £5 and transfer her number to that and then take the above deal out? By the time it's all sorted we should be able to transfer the number from Lebara back to Three?

Any advice greatly appreciated

 

[MRBILLYUK]

My g/f is on Three but got a new phone on Three through fonehouse. They had better deals on there than going directly through Threes upgrade. She got a new sim and number which she had to use for a week before her old number was migrated over onto her new sim. It was a bit of faffing about but it was a really good deal.