Detailed Answer
Retrofit guard installation on existing gutters is standard practice and the majority of what professional installers do. The key prerequisite is that the existing gutters are structurally sound enough to hold the guard system and continue draining correctly.
Before any guard installation, a thorough inspection of the existing gutter system is essential. Installers check for proper slope (1/4 inch per 10 feet of run toward downspouts), secure hanger attachment without sagging, and no rust-through, holes, or failed joint seals. Gutters that pass this inspection are solid candidates for guard retrofit.
Guards should not be installed over gutters that are sagging in multiple locations. A micro-mesh guard installation doesn't correct underlying slope problems — it covers them up. If the low point of a sag becomes a debris accumulation zone under the guard, you'll end up with a clog you can't see or easily clean. Address structural gutter issues first.
Rust-through or significant joint failure also disqualifies retrofit. Guards add weight and, in heavy rain, add water load to the system. A gutter section with a rust hole will fail faster with guard loading, and the guard will trap the evidence until the damage is severe. At that point, the guard has to be removed to access the gutter anyway.
For gutters that are 15–20 years old and in otherwise good shape, guard installation is often the best investment. Guards reduce the maintenance burden for the remaining life of the gutter, and many homeowners then do a combined gutter replacement and guard installation when the gutters eventually reach end of life. The guard cost is a separate consideration from the gutter replacement cost.