There is certainly an evolution, or decadence, depends on the point of view you take in the Roman army from its peak of effectiveness and fame to its final demise as a fighting force. We are always talking about the Western Roman Empire, the Eastern part known to us as Byzantium remained an effective fighting force more several more centuries.

Now, the classic legions we know were based on strict discipline and training to fight as a cohesive unit, the Legion (as it actually refers to the whole Roman Army) suffered several reforms during the different periods of the Roman Empire. There is certainly an evolution and a change from organic organization to changes in tactics and strategies. The expansion of the Roman Empire also stretched its forces having more and more frontiers to guard and territories to keep under control.

Among the evolution, to name few, we can see changes in the Army structure with increased use of Auxiliary troops, with issues of manpower hence being able to admit into the ranks "barbarians" which in turned would bring a change in equipment and ultimately in tactics and training. Similarly the classic Legion stationed in a remote frontier would give way to the defence in depth concept where mobile reserves would be concentrated in strategic points from which to go to the hot spot on demand; this would also require the raise to prominence of cavalry in order to fulfill the mobility concept while past units were traditional heavy infantry units.

To summarize , yes, there was a change in the Roman Army and from my humble perspective a decline, one the best books I know for the general public on the matter is:

The Fall of the Roman Empire: The Military Explanation" by Arther Ferrill, Do not take it as the only explanation to the fall of the Roman Empire but describes very well the Roman strategy and how it evolved reacting to social changes and the threats in the frontiers as centuries went on, if you get a copy you'd get a lot of clarity to your question.

http://www.amazon.com/The-Fall-Roman-Empire-Explanation/dp/0500274959

PS: I see in Amazon they have used copies for as less as 1 cent!!!