Thats an interesting concept and reading books like Mark Urban's 'Rifles', it seems that green troops can gain combat experience and battle savvy over several months of active campaigning.

However, a period of rest and recuperation can undermine that willingness to go back into battle and take the edge off their abilities particularly if their situation is particularly comfortable and free from danger.

The book also makes the point that individual soldiers and thus the entire regiment can peak and begin to decline as a result of over exposure to risk and danger. Some veteran soldiers can begin to feel that they have done enough and that their luck cannot continue to protect them for much longer if placed under further demand, apparently Wellington was amazed to discover that many veteran Riflemen were deserting rather than go back on campaign. Others can decide that survival is no longer a possibility and that death is preferable to a continuation of their suffering.

Another danger is that veteran soldiers can begin to form a theory of invincibility that causes them to ignore the risks of battle. This was true of the airborne division during operation Market Garden who convinced themselves that they could achieve success regardless of the risks they were being asked to undertake. I met one veteran from that battle who told me that he and his colleagues were so charged up by that time that they could not be trusted in towns and were made to pick potatoes in rural area's between jumps to try and get them to calm down. There favorite party trick apparently was to play chicken with flamethrowers, seeing how close you could get to the flame without flinching.