There's no such thing as a 4G SIM or a 5G SIM, you get EE SIMs which are suitable for PAYG, pay-monthly, 4G or 5G etc - it is your plan which allows for access to different network services.
How did you obtain this new SIM? Was it on the EE website or a third party? I'm assuming you will have received a cover email or some paperwork with it - does it confirm that it's a new connection?
When you change from a pre-existing EE pay-monthly plan to a SIM-only plan, there is no reason to replace your SIM and the account change is often immediate.
