The hardware architecure of the NAVSTAR Space Segment is, for all practical purposes, fixed. In general, hardware can be upgraded by either replacement or on-site component replacement. Neither of these is feasible for the NAVSTAR Space Segment. Satellite launches are expensive; it would be prohibitively expensive to replace any significant number of SVs for other than catestrophic failures. It is not feasible to perform on-site service on the SVs due to their orbital altitude and the scarcity of Space Shuttle missions.
For related reasons, the software architecture of the NAVSTAR Space Segment is also largely fixed. The majority of the SV software is real-time control software; as such, it is largely hardware-dependant. Since the hardware is fixed, the majority of the on-board SV software must also be fixed. While minor upgrades, bug fixes, and minor improvements will doubtless be made to SV software as required, major redesign would require remote replacement via radio link. Even if possible, the risk involved (potential irrevocable loss of one or more SVs due to load failure/boot failure/undiscovered flaw) would likely be deemed prohibitive.
The NAVSTAR Space Segment has two communications architectures: Control Segment - SV and SV - User Segment. As discussed above, neither of these architectures needs redesign at this time. The SV - User Segment communications architecture is standardized, forms a hetrogeneous open system, and needs little change at this time. The risks involved in changes to the SV portion of the Space Segment communications architecture are prohibitive and should not be attempted.