Simplifying flies is always risky. It is safe to assume the original pattern included essential materials in the pattern. However, I feel the features of the materials often outweigh the amount of material used. Thus, it is often possible to reduced the steps or amounts of certain materials, but maintain the specific and often essential quality in the pattern. For example, a Half Back fly includes a second palmered hackle over the anterior portion of the peacock hurl body. However, I find that the flies without the anterior hackle catch fish equally well. I believe the anterior hackle is unnecessary because the qualities of rear palmered hackle are enough to create the right features in the water.
Alternatively, I can take an existing pattern and add materials or steps to the tying process that produce a more reliable fly (at least until its actually tested on the water). These modifications often take simple forms. Brass beads are common additions to many classic patterns, but I favor colored glass beads too. These beads come in many different colors and can add either bright contrast or more subtle accents to a common fly. What is most important is that the pattern now looks slightly different from the common pattern thrown by most others on the water.
Again these choices can be risky, but often prove a rewarding solution once tested on the water. I admit when I substitute it is often because I lack the exact material called for by the pattern recipe. Sometimes these choices work out and some times they do not. For example, there is no substitution for rubber legs on a Bully’s Bluegill spider. While other leg materials can be tied on the fly, no other material has the appropriate stiffness to act correctly when stripping the fly. Silicone and spanflex are both too soft to return to the open standing position necessary to create the parachute like descent in the water that drives blue gill crazy.
When I make the decision to change a pattern I usually tie about 5-6 for my fly box. If the flies prove themselves I will add more. However if they do not act as exoected in the water or failed to produce I remove them from my flybox. However, with 5-6 flies, I ensure have enough flies to properly test it on the water (and leave a few in trees).
You have to be fearless about tying flies. Often changes to flies prove ill advised, but every so often a tier stumbles into really amazing flies by simply taking at shot at something with a bit of unique flavor.