Hi asmith161,
Thanks for posting on the community, apologies for our delayed response.
I'm not too sure if you've been able to get some help with this since posting, but we would be able to swap your existing and new SIM around, and then upgrade your existing SIM to the Oomph tariff (with the number you wish to keep) and then cancel the new SIM. We wouldn't be able to issue/use a PAC in order for you to keep your existing number as you are not switching between two different networks, if that makes sense.
Let me know if you are still seeking help with this and I'll send you over a PM.
Thanks,
Beth