You are partially correct however you don't need to port your number to another network, there is another way.
What you need to do is give Vodafone thirty days notice to move your current contract onto PAYG. Wait until around the third week of this notice, (day twenty one or so), and then order your new deal from the third party provider.
When you receive the new handset from the third party supplier get it up and running making sure everythng is okay. Importantly, do not register a new account with this number on My Vodafone yet, if you port a number across to this account it can be problematic changing it later.
On day thirty your original contract will become PAYG. It can happen any time of day but in my own experience it happens quite early on, now your original contract with Vodafone is finished. If you still wish to have access to your online account relating to this number it's important to have another phone number registered on the account prior to this happening, otherwise there is no way to receive the text message with the six digit number.
On the same day fill out the form to transfer your old PAYG number to your new contract.
https://vodafoneinlifeporting.co.uk/ilp/web-forms/pay-as-you-go-to-pay-monthly/
This should happen next working day but transfers don't happen over the weekend.
The next working day your number should be transferred to your new contract. Once the transfer has happened you may need to turn your phone fully off and back on again to complete the process, rebooting the phone is not enough.
Now you are free to use your new contract with your old number happy in the knowlege that for the small amount of effort you are now getting much better value than upgrading directly with Vodafone. Also you can now register an account with your number on My Vodafone but you must use a new email address in the process.
