Plan change

Do I have to return the modem and pick up a new one when I either downgrade or upgrade from current internet speed to new speed? ...

If the device you have now supports the new speed there's no need to switch. See the "Maximum data throughput" ratings on https://www.xfinity.com/support/articles/broadband-gateways-userguides.

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@Luckyucsd If you rent a modem from us, you should be fine. If you provide your equipment, you will want to look for your equipment on our preferred devices list here: https://www.xfinity.com/support/devices/#unauth