TL;DR
Rehab a Utah well when the casing is sound, yield loss is under 50% of original, and the water right is locked to the location. Drill new when the casing is breached, yield has crashed by more than 70%, or the right can be moved to a better spot on the parcel.
Summary
A practical decision framework for Utah well owners weighing rehabilitation against a brand-new well — yield thresholds, casing condition, water-right status, and the math that decides it.