Upsell will be an upgrade key. When you purchase the license an upgrade is different from a full license. I guess they only allow the standard to enterprise direct upgrade with an upgrade license. If you just have a full CF 11 license then you will need to reinstall and use this key on installation.
True I needed to re-install CF11.
I'm late to the party here, but I found I was able to edit the license.properties file (in the [coldfusion]\[instance]\lib folder, where instance is cfusion, if you have only that one default instance), and I copied the Standard key from the sn= value to the previous_sn= value, and then put the Ent key in as the sn= value. On restarting, my CF Std had switched to CF Ent. This was in CF2016, but I suspect the same would be true for CF 11.
And while it's a gray area talking about circumventing licensing warnings, I would say that if in both our cases we had a legit Ent key to go with our legit Std key, then swapping them is legit and should be as easy as dropping in the new key. Since it is not, this workaround seems reasonable.
I will say in closing, for anyone else who comes upon this and may try it, be sure to save a copy of your license.properties file before editing it, so that you could revert to it if needed. (Again, you would need to restart CF for any changes to take effect.)