I recommend this book:
You might want to read some of the articles on this website about western saddles & fit: False "saddle fit rules" regarding the shoulder blades
There is no reason to switch to a western saddle & western saddle tree if one rides it 'English'. It can be done, but it will create an unbalanced saddle on the horse's back. Since the weight distribution system (the tree) is different, one's balance in the saddle should also change.
The norm in western riding is to drop the idea of 'contact' and headset. Ride with slack in the reins, and let the horse choose its head position. You can direct rein still if you wish, but the goal is to transition to using one hand. With a curb bit, moving your one hand up, back, left, right - without taking all the slack out of the reins - will communicate your goal to your horse...once trained.
As I've started riding in a western saddle this last month, after years of Australian or English saddles, I find it is more of a platform to do things from than a saddle to feel the horse. The impression I get is that I need to change my approach to balance in order to use the western saddle to its full advantage. One thing that surprised me is that my horse seems quite content with the extra weight and length of the western saddle. She turns around a pylon faster and with more freedom in her shoulders than when using the Australian-style saddle.
After a month or so of intermittent use (bad weather and other commitments), I find it to be very different than riding English, just in terms of balance. I had switched to a western approach with the reins a couple of years ago, but the change in saddle gives it a very different feel. I'm still not sure if I like it or not.