I've been back on FiOS since we moved back to PDX last summer, so my last cable modem switcharoo was about 5 years ago in Seattle. It's likely they could've made it easier.

To that end, I've done the modem switch probably a dozen times for myself, family, or friends and I've never encountered any resistance from Comcast during the process. Though they did fat finger a MAC address or serial number one time.