ArenaNet Forum Chat for September: The next forum conversation, on the topic of our September 18th release -- including Living World Season 4 Episode 4, a new raid, a new legendary, and more -- invites member questions and comments from Tuesday (9/25) through Friday afternoon Pacific Time (9/28). Devs will be responding through early October, as their schedules allow. Join the special September 2018 AFC Subforum to take part. More info here.
At the moment this is sometimes possible by dismounting your current mount and then quickly mounting up with your other hotkey. However it's not very consistent, it may even just be a bug.
I would like to instantly and seamlessly switch mounts, and I can't think of a single reason why we shouldn't be able to.