Sorry to ask, but how experienced you riding english? If you don't have that much of experience, and never worked with knowing nothing horse I'd suggest to go to the trainer (preferably dressage one), so both of you could progress together.
P.S. You absolutely can switch to English with the trail horse that knows nothing. Both my horses (qh and paint) were trail riding mostly with no formal training. Then I decided to go into English riding, started lessons with the very good dressage instructor, and the changes were noticeable already in 2-3 months (mind you I knew nothing about cues and all when I started
). After 7 months of lessons with my qh we also started showing dressage on low levels. So yes, it's possible!