A string-replacement assembly may be installed within an inner cavity of a body of a guitar or non-fretted string instrument. The assembly includes at least one cartridge configured to contain a supply of replacement string therein, corresponding to a particular string gauge for the guitar. Each supply of replacement string is rotatably mounted within a cartridge. A free end of each supply of the replacement string can be threaded through an exit opening in the body of the guitar for installation on a topside of the guitar or non-fretted string instrument. When a string needs to be replaced, a portion of the supply of the replacement string may be readily advanced from a corresponding cartridge to a desired length, and installed in a playing position on the topside of the guitar or non-fretted string instrument, to replace a removed portion of the string.