Welcome to the forum and thanks for your post.
Is there any particular reason why you want to swap the numbers about? As you're keeping both the numbers would you not keep things as they are? Personally if it was me that's what I would do.
I'm not too sure if it's possible to do what you have requested. Someone may be along to offer further advice on this.
Thank you for all the replies. Sorry I have been out of contact for a while. The reason I wanted to change was to keep old number on my new unlocked phone using my BT sim. I was being a little silly really as nobody knew my new BT number. The obvious answer is just to puchase a new EE sim for my old locked EE phone.
In order to keep both numbers you could transfer the first number to a third-party SIM, e.g. Giffgaff, then transfer the second number to the SIM for the first number, then finally transfer the first number from the third-party SIM to the SIM for the second number. Bit long winded and might mean spending a few pounds to register a third party SIM, but would achieve what you want. HTH.