The original idea of these OS maps was to act a prototype for the eventual production of linear maps of each long distance footpath. Unfortunately this idea was abandoned by the OS (with the exception of Hadrian's Wall) for a number of unexpected reasons.
Firstly, the take up by walkers was, surprisingly, well below expectations. This was eventually traced back (through a questionnaire) to the fact that walkers, in general, like myself, tend to undertake LDPs using a guidebook. These invariably contain a linear map so the standard OS maps are taken as well to provide an overview of the terrain beyond the linear limits. They saw no reason to purchase an extra linear map.
Secondly, the choice of the C to C as a first trial was a poor one in that it was an unofficial trail. The route had been plotted without consultation with interested bodies and received little support from NPs or councils. AWs route was perceived as paying no heed to local environmental needs. There were issues (such as Nine Standards) where the route was unsuitable for year round heavy usage and subsequent diversions were required. The OS found itself caught in the middle of an unexpected controversy.
Thirdly, stemming from the above, the number of minor, but significant alterations (as around the Lion) would have meant frequent reprints which sales could not justify. The idea of linear long distance routes therefore bit the dust.
My mention of Rupps, BOATs and RBs was intended to show that OS maps are an evolving system which constantly change. The longer a map is held the more it becomes out of date. The OS were always reluctant to include RTs on maps as their status had been occasionally challenged. However a court case recently clarified the law and they are now starting to appear on maps, although unnamed. These could significantly assist walkers.
Yes, you can still use a ten year old map and you will probably not experience too much difficulty. However, as in all cases of people walking in remote terrain, the more up to date the map you have the less likely you are to encounter problems, especially under adverse conditions. Don't rely on luck, always consider the worse case scenario. Compare your map to the ones in a modern guidebook or current OS maps (viewable on the OS website) and make the decision yourself from the evidence.
Here endeth the first lesson!!!
Agreed, most of a map will remain the same. However, advise is always to use the best equipment that you can afford. Most walkers carry decent waterproofs but I have frequently seen individuals shelyering from a storm clad in a poncho or pacamac.