Ah, this is one of my most popular FAQs!
In most cases, my preference is to go with a saddle that is able to be adjusted. Horses change shape frequently with workload, age, type of work, seasons etc so having the ability to adjust the saddle as these changes arise allows us to continuously achieve a sympathetic fit.
Is my saddle able to be adjusted, and if so, how much?
Different brands and models of saddles have different amounts of adjustability. Regardless of how adjustable a saddle is, if the tree profile, panel design, shape, style and features are not suitable for your horse then you will never be able to get a perfect fit. Narrowing or widening and flocking or shimming adjustments are tools to achieve a sympathetic fit but can only get you so far!
Some saddles are built with an interchangeable gullet bar meaning that you can remove the width bar from the saddle and replace with another width. This is a good option when you know that the tree shape and saddle design/pattern is a suitable shape for your horse but know that he fluctuates in condition. An interchangeable gullet bar is DIY although be sure you are adjusting it to the correct size if doing this yourself. Other saddles are built on trees that are able to be adjusted by a saddler, fitter or retailer. This type of adjustability can give you a more precise fit as the widths can be adjusted a full size, half a size or to a horses template.
Another adjustable feature is the panel of the saddle. Saddles with wool flocked panels offer a huge amount of adjustability as your saddle fitter is able to rebalance and customise the fit by flocking the saddle as needed. Saddles with latex, felt, rubber etc are not able to be rebalanced but you can balance the saddle outside the confines of the saddle with pads or shims. The CAIR system allows you to add shims into the panel to lift the front, middle or back. The FLAIR system is able to be inflated in different areas to customise the balance.
Will my saddle fit my horse forever?
This cannot ever be guaranteed. If your horse changes so that the saddles features are still suitable but a wider or narrower fitting is now needed due to changes in condition then adjusting the width of the gullet is a simple solution. If your horse changes in muscle tone, fitness etc then your saddle fitter can rebalance your saddle to perfect the weight bearing and distribution either inside or outside the confines of the saddle.
If any major changes occur in your horse then you may need to reevaluate your saddle situation. For example, if you purchased the saddle when the horse was a green 4 year old and he is now a fit, strong 10 year old it is likely his needs in tree shape and other features may now differ. Seeing into the future is impossible but your qualified saddle fitter will do their best to fit you and your horse as seen on the day and also try to "future proof" where possible with the saddle they choose.
The only guarantee your saddle fitter can give in regards to long term saddle fitting is that if you keep up with regular routine saddle fittings they will ensure your horses comfort long term however is necessary: adjustments, alterations or changes.