In the time I've had him (a year and a few months now), my gelding has changed shape COMPLETELY. From straight backed and in a red Wintec gullet plate (which is a wide), to slightly swayed and in a black (medium) gullet plate... my dressage saddle doesn't fit him any more! It was perfect, had it professionally fitted and all, but now it's bridging.
Hopefully my tax return comes through soon so I can get him a new dressage saddle... in the mean time I'll just ride him in my CC, because that still fits. But you know, I only bought that a couple of months ago, and it had a medium plate in it when I got it which was WAY too narrow and the saddle was perched on top of him... switched it out for a wide, and then a medium-wide, and I just checked the fit on him today and it was too wide... stuck the medium gullet in and it was perfect.
Moral of the story, make sure you check the fit regularly... horses change shape very quickly, quite significantly sometimes, and a saddle that was perfect a month ago might not fit now!