How to switch from bosal to snaffle bit?

I have a 5 yr old rescue mare that is great in a bosal(backs,stops,neckreins etc) but when introduced to a large snaffle starts tossing her head and tries to evade the bit in general. She also shoulders out on the lunge line and tries to do so when I go in circles when I ride her. I have tried "bitting her up" do to the head tossing and the stiffness issues with abhorence to the people at the boarding stable I have her at who have made it clear this is a very cruel practice. I had her very loosely tied for maybe 2-5 minutes each side where once she flexed she had relief from bit pressure-but was pretty much made into being an abuser.I try to ride her into circles and have her flex her head when I am on back-with lots of head tossing-when I was "bitting her up beforehand" without the criticism, she seemed to be more flexible and less evasive. Any new ideas for me to help this horse along?