By replacing key elements of the circuit with components of a different value, you can alter the sound and intensity of the effect. For example, by replacing a 100K resistor with a 120k resistor, you increase the resistance against the flow of current through the circuit; in a distortion pedal, this tempers the distortion. Conversely, reducing the resistance against the current flow typically increases the intensity of an effect.
Transplanting is the process of installing a complete circuit into a new chassis, typically to either alter the function of the circuit, to salvage a circuit from an otherwise damaged pedal or to take advantage of the superior build quality and chassis-mount components. For example, a wah pedal and a volume pedal both feature a treadle, which attaches to a slide potentiometer. A wah pedal is a frequency filter effect; the position of its potentiometer governs the frequencies that are heard. By fitting a wah circuit to a volume pedal chassis, you effectively convert the volume pedal into a wah, maintaining its appearance but completely adjusting its function.
A rewarding alternative to purchasing guitar effects is to build a pedal circuit based on the design of a popular pedal. The circuit, typically comprising diodes, inductors, potentiometers, transistors, capacitors and resistors, determines the sound of the effect. The parts are usually of equal value and function, but may be of inferior quality. For example, whereas a major branded distortion pedal uses premium resistors and capacitors, a clone project may use cheap imports. The sound of the finished product is very similar, but the cheaper products may lead to a decreased life span for the pedal. You typically build a clone pedal from a self-assembly kit. These come with made-to-measure parts and a wiring schematic.
By designing and building your own circuit, you can make a completely unique pedal. This type of project calls for an advanced understanding of circuits and the effects of the individual components. However, by studying the schematic of a pedal that you like, you can create your own interpretation of that unit by designing a similar, yet distinct schematic. Schematics for commercially available and clone pedals are typically published on Internet databases by hobby builders. The degree of uniqueness is up to you.