How long does a plan upgrade take?

Just to close this out - The upgrade should generally take effect immediately, no significant delay. It took some work, with Xfinity support on DM, but I was finally able to determine that I was actually getting the expected speed at the modem. My old Netgear 802.11N router was the choke point. The moral of the story is that just because a router has gigabit Ethernet, that doesn't mean that it can route IP packets at gigabit speeds. I replaced the Netgear router with a newer ASUS 802.11AX router that I had been using as an auxiliary access point and now I'm seeing 450 Mbps at the PC.

Useful information I had to learn to discover what was going on: A modem that doesn't have a built-in router puts its Ethernet port(s) directly on the ISP's network. That is, it behaves as if you had a very long Ethernet cable plugged into a switch at the ISP's network hub. To connect a computer directly to the modem, the computer has to be set up to get its network configuration by DHCP, and the computer will then be directly connected to the Internet. This is, of course, a really bad idea and should only be done very briefly when absolutely necessary for diagnostic purposes, and if your computer doesn't run its own firewall, it would be best to fix that deficiency before connecting it directly to the modem.

I just went through the same situation ordering an upgrade from a 100 Mbps to 600 Mbps plan. My new Motorola MB8611 modem was already working at my old speed of 100 Mbps. I waited and waited 4 days for my new speed to show up but nothing happened.  On the website, when I went to My Account and looked at the Services | Internet box, the Manage Internet link was missing, replaced by "Waiting for service"(not sure).

I know enough about cable modems to know that the modem needed to be "reprovisioned" with a configuraton file to configure my modem for the new speed. I assumed Comcast would initiate this automatically. It shouldn't take that long. But the reprovisioning never happened. I started poking around the website to see if I could trigger the reprovisioning myself. I couldn't even reset my modem from the Comcast website. Note: This didn't make any sense. A fully functional DOCSIS modem gives the cable operator the ability reset, read status, and configure the modem. And my modem looked like it was working. I couldn't even read status of my modem. All I got from the Troubleshoot link was "We're having trouble finding a signal".

So I decided to try "Chat with Xfinity", a fully automated help program. When I told it my speed was too slow, the program detected a problem and asked if I wanted to reset my modem. With nothing to lose, I said yes, not expecting anything since none of the other "Reset my modem" links worked. But amazingly, my modem actually reset! The Chat program said the reset would take 12 to 15 minutes. Huh? I watched the lights on my modem and it took only about 3-4 minutes to resync all the carriers. I told the Chat program my reset was done but it told me to wait. It mentioned updating my configuration. What!?!? That is exactly what I wanted!! As soon as Chat told me my reset was done, I ran a speedtest. 610 Mbps!! Awesome. My modem was finally provisioned.

Bottom Line:

If after requesting a plan upgrade and not getting your speed upgraded, use CHAT WITH XFINITY and ask for a modem reset. It will reset your modem and get you provisioned properly.