Thanks for your inputs, guys!
I definitely understand the saddle being too narrow now. I'm going to try and get a saddle fitter out soon.
When you say that the foam panel CC saddles rarely fit, what would you suggest being a more forgiving fitting saddle for her to have?
I was looking at the gullet change saddles (Wintec, Bates) and the CAIR flocking saddles thinking they might give a wider range of correct fitting. What would you all recommend me to look into?
Would she only need the next size wider? I'd be going from a regular tree to a wide tree. Of course, I would still need to check the fit after getting it, just in case.
I think everyone has their preference. I think most will agree that flocked panels are very nice, not only in the support they provide, but also because they can be adjusted by a saddler for a custom fit to your horse. I like wool-flocked, but I'm old-fashioned. I know people like synthetic flocking too. Then there's the modern foam, which is not the same as a close-contact panel. Then there's CAIR, which is loved or hated by riders or horses. Then there's Flair (adjustable CAIR, basically) which seems to be more loved than CAIR.
For the size, I think you should take a wither-tracing. Use a flexi-curve or coat hanger to get your horse's shape about 2" behind the rear of the scapula. This is about where the tree points should sit. Trace it on a piece of carboard and cut it out. Preferable take your tracing with you shopping. If that's not possible (ie buying online), then take a look at the angle where the tree points will sit. If you're right about 90 degrees, then around a medium, more is wider, smaller is narrower. You also have to consider the profile lengthwise along the back.
Generally, I'd say that if you currently have a medium tree, you'd need a M/W or wide in that same brand. Every brand is sized differently. You almost start from scratch every time you try a different brand.