What are the most critical differences between a well-executed wheel upgrade on an older bike frame versus a modern frame, in terms of achieving optimal performance and minimizing potential complications, such as brake caliper clearance issues or frame material compatibility problems, and how do these differences impact the overall cost-benefit analysis of upgrading to fast road wheels, especially when considering factors like frame stiffness, tire clearance, and the bikes original design intent, and what are some common pitfalls to avoid when attempting to upgrade an older bike, such as overlooking the need for a headset spacer or failing to properly re-dish the rear wheel, and how can riders effectively weigh the importance of factors like aerodynamics, rolling resistance, and durability when selecting fast road wheels, and what are some key considerations for riders who plan to use their upgraded wheels for both training and racing, including the potential need for a second set of wheels or a wheelset with a more durable construction, and how do the unique demands of different types of riding, such as criteriums versus time trials, influence the choice of fast road wheels, and what are some effective strategies for optimizing the performance of fast road wheels in real-world conditions, including techniques for minimizing the impact of crosswinds and maximizing the benefits of aerodynamic wheel designs.