The answer to 'difference between system upgrade and' is that a system upgrade involves replacing your entire HVAC system with a newer, more efficient model, while a system replacement simply replaces the worn-out components of your existing system. Upgrades provide greater energy efficiency, improved comfort, and longer-lasting performance, but come at a higher upfront cost. Replacements are a more budget-friendly option that can extend the life of your system, but don't offer the same level of efficiency and technology improvements.
Factors to consider when deciding between an upgrade or replacement include the age and condition of your current system, your home's size and cooling/heating needs, your budget, and your long-term goals for the home. An HVAC professional can assess your situation and recommend the best solution.