Actually, I would be more worried about putting a western saddle on an english horse.
All my western horses took the englishs addle as, "Thank GOD, a little less weight to carry!"
However, I did a full-care lease on a Hanoverian gelding last summer for a jumper circuit, and when I put the western saddle on him ti was like "OMG Mom put a big giant thing on my back!! WTF Do I do??" And then we had a big episode fo him trying to jump in my lap.
After that, I went to Toni's and saddled some of her english horses western (She likes versatility) and they were also not too thrilled. None of them ended up in my lap, but they were a bit disturbed by that giant thing laying across their backs.
Not saying all english horses will react like this, but apparantly all six of those horses do.
The english horses aren't used to a saddle covering that much of their back, while western horses are just used to weight.
Transitioning shouldn't be hard. I wouldn't worry about it.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.