It looks like my folder was not an Encrypted Folder to begin with (ie created before Sync 2.3 release), so there is no option to generate an Encrypted folder key/link. The only way I could get the folder to migrate to an Encrypted folder is to disconnect both ends, create new Sync folders in the same directory. However, this caused all of the data on the server where I want the Encrypted Folder to get overwritten, and have to Sync all over again.
I tested this migration on a smaller folder with less critical data, so this was fairly painless (and took ~10 minutes to transfer). However, I'm nervous about doing it on my other folders that are larger and have more sensitive data that we can't afford losing (and don't want to have to wait for all of the devices to re-sync everything). Is there any way to add an Encrypted Folder share key to existing Standard or Advanced folders?