Shanklove, thanks for posting these awesome questions on the forum!
First thing you want to do is check to see if the new XR device is compatible. Please use this site: https://www.xfinity.com/mobile/byod/compatibility/device to check.
Once you've done that, activate the device (details on the site above). If you want to swap the lines from phone to phone, the only way to do that is by swapping the sims, as you stated. We do not encourage swapping sim cards. Sometimes, provisioning for one device is tied to the sim so you may come across feature issues if you swap the sim. But if you decided to swap the sims (from one eligible device to another), you should be able to make phone calls, browse the web, etc. If you come across feature issues like voLTE or voicemail, you can attempt to replace the existing sim cards with new replacements with no provisioning history.