@JMA

If you check the official manuals for the years 1905-14 you find that the Boer war and the Russian/Japanese war had indeed an impact. However, only for aspects on side of the defender: Better field fortifications, hand granades, etc. ...

The crucial question, how could the attacking force overcome this improved defense was not answered, despite the fact that the war of 1904/5 already showed most of the problems the attacker faced when there were long fortified lines.

The impact of veterans depends on the relevance of their knowledge, if they provide the relevant stuff they are a kind of force multiplier, if their knowledge is not longer relevant they may even become a problem when being higher commanding officers.

Military history shows all facettes of this problem.

The BEF had 1914 IIRC a much higher percentage of officers and NCOs with combat experience compared to the French and German forces, did this translate into better performnace? To my best of knowledge the first attemps -after half a million KIA/WIA on each side - to develope small unit tactics that allowed attacking fortified enemies without crippling losses were made by French and German officers in 1915.