switching from Three to iD

Hi

So I decided to move from Three to iD mobile. I complete an online application to do so and as part of that application I am asked to give my PAC as I want to keep my old number. I am asked to set a contact start date and I choose a date a few days into the future, from memory the 4th June 23. I click submit.

My SIM arrived, I assume this is linked to a new number I don't need. 

Q1. Will the old Three SIM just stop working some time on the 4th? 

Q2. Will the new SIM then have my old number when I slot it into the phone?

None of the info I have been given by email or in the leaflet that came with the SIM make this clear, there seems to be no recognition that you can give your PAC when you make the online application! 

Thanks