Thanks for taking the time to post! Gladly, I'll provide further details.
It is possible to SIM swap to your unlocked Pixel 3a but your Pixel 3a will reflect as an unsolicited device. Click on the following link so that you can check and confirm that your Pixle 3a is compatible with XM service - https://www.xfinity.com/mobile/byod.
If the device checks compatible, we can put in motion our BYONDEL (Bring Your Own New Device on an Existing Line) program and we can officially update the device type on your existing mobile number.
Let me know of any other questions that you may have.