I have a Bates Caprilli saddle and its around 6-8 years old (I've owned it for 4 years). It takes me about 5 minutes to change. I'm not a saddle fitter, but I have been able to go to a wide cob (with the x-wide gullet) to a finer boned BRP x welsh (medium gullet) with no issues - it fit them both well. Of course only the gullet changes sizes, not the channel. I bought it as my ponies were young, and different sizes so I thought I could always go up a size when they bulked up.
Depends on the saddle. My parents have two bates, same model, bough within a year of each other. One can be changed in about 5 minutes by one person. The other requires two people (preferably strong people) and a lot of grunting and swearing. I found the pesso's to be MUCH easier to change
The gullet plates do gradually get "flattened" with regular use.
My coach also found that she needed to have a saddle fitter actually shape the gullet plate that fit her mare best so that the ends flared out a little bit and stopped restricting her shoulders. It was easy enough to do, and she probably could have bent it herself, but she was afraid of snapping it so had a professional do it.
Ditto this, the first 2-3 times.... I was contorted in several different ways. Foot holding one flap up, elbow acting as a lever, while the other arm attempting to get the plate in place and line it up properly. Just UGH!
Now it isn't too bad, though I wouldnt want to do it every day,