Time to Upgrade Modem or Not?

I currently have the TP-Link TC-7620 DOCSIS 3.0 (16x4) with max speed of 686 Mbps.  Receiving emails that Xfinity internet is now giving speeds up to 1.2 Gbps in my area.  When I do a speed test the result is 494 Mbps.  Do people here recommend a modem upgrade yet?  It looks like the upgrade is not yet necessary since the download speed is still below my modem's max of 686 Mbps.  Xfinity emails state "Your current internet equipment is not able to deliver these new speeds."  In other words, if Xfinity was actually hitting 1.2 Gbps it makes sense that I would be missing out on this due to my modem's max speed being below 1.2.