A remote measurement unit for testing and conditioning one or more telephone lines includes multiple electronically erasable flash memory banks, which contain respective versions of the operating system employed by the test unit's micro-controller. An operating system modification routine employed by the host processor of a remote site allows the functionality of the remote test unit to be selectively modified by electronically installing an upgraded or downgraded version of the operating system, or by electronically selectively activating or deactivating one or more operational features of the currently active operating system. A reset routine ensures that the operating system modification mechanism will only boot up the `correct` one of the two quasi-redundant systems available to the RMU's microcontroller after a system modification has been performed. In addition, the temporary insertion of a no-op code field just prior to the beginning of the program prevents the RMU's processor from accidentally booting up a modified system until the system has been verified as operational.