I remember Caesar in De Bello Gallico mentioning Romans changing fighting/resting troops in some siege defense while Celtic attackers didn't do this and lost the battle because of it (it was somewhere in Helvetia I think). This is just one mention but I'm interrested in the whole phenomenon of changing first line troops in the battle throughout history (up to Middle Ages).
I know this is very broad topic and might lead to long discussion/argument (I have seen this before on few living history forums), so I prefer links to sources on this topic to reasoning.
EDIT: I've heard a lot reasons for and against changing first line troops not only during siege but in open-field battles too, but I don't remember any relevant quote of historical sources other than the one mentioned, so I'm interrested in either primary sources or qualified articles/books on it (in both sieges and other battles).
To sum up the reasons:
If my sources are true, there were very low casualties in most ancient battles (see this question for details). One explanation of this fact claims that armies often stopped to allow retreat of wounded/exhausted soldiers, and respected their enemies' right for such a retreat to some extent.
Fatigue. Even historical military reenactors vary widely in estimations how long can someone fight in a full pace until he has to slow down, but if one side's soldiers can switch between fighting in high pace and resting, that side has a big advantage.
- Any temporary break in the shield wall may lead to big casualties or even losing the battle. If the enemy pushes hard, changing first-line troops is impossible, and it's just risky other times. Without being well-trained for such a maneuver or without enemy making a short break every X minutes, this risk is much worse than ill effects of fatigue.
This is a sort of a summary of the major points of the other discussions I've read. I have asked a related question for the casualties subquestion.
EDIT2: the question again and more clearly (I hope): is there any book, article or other source on changing fresh/fatigued soldiers in battle?