An “extension ready” spinal support system that enables the extension to be accomplished with minimal disturbance to an existing spinal support structure to which the extension system is coupled. In some embodiments, the existing spinal support rod and pedicle screws can remain intact while extension subassemblies are mounted directly to the existing base rod receptacles. The extension subassemblies include a skirt portion that surrounds and engages the existing base receptacle to prevent splaying of the base receptacle. Additional resistance to splaying may be provided by a canted thread arrangement between the skirt and the base receptacle. In some embodiments, the extension receptacle is provided with a low profile (i.e., shortened axial length from the base rod receptacle) by providing a monoaxial rotation structure that rotates about but does not pitch relative to the extension axis and is shorter relative to polyaxial rotation structures.