Just to be pedantic...
'Navicular disease' is effectively the result of bad mechanics. It affects the whole hoof. Getting hooves functioning well(generally with sick feet I believe this is best unshod *not nec. bare*) will correct the problems that cause it & therefore halt & somewhat reverse *the symptom* of 'navicular disease'.
It is generally a very slow progressing 'disease' and while of course it's best to correct any problems ASAP, pretty much whatever stage of the 'disease' actual rehab is started it will improve the situation. It doesn't have to be done quickly or not at all. While if damage has progressed to bone loss/remodelling, that may not change, if hooves are functioning efficiently, that doesn't appear to matter. Horses can still be made sound. Many 'navicular' horses arrive at 'barefoot rehab' only after years of conventional methods have been exhausted.
I wouldn't wait until the problems progressed to bony changes before 'treating'. Xrays can only really tell you how far it's gone & absence of bony changes doesn't mean absence of the 'syndrome'. If the horse is weak heeled & so landing toe first(this is a big cause
to stress/damage on the navicular region, rather than just product
of the disease), long toed/low heeled, etc, I'd be looking to correct this well before it progresses to that stage.