Leave/let well alone
Definition 1 of 1
Pro Tip 1/3Context Matters
Use the phrase where change might lead to trouble or complications in a situation. 
Your garden is thriving; better leave well alone and not add new plants.
Pro Tip 2/3Risk Awareness
Apply the phrase when it's risky to alter something that's already satisfactory. 
The contract looks good as it is; we should leave well alone.
Pro Tip 3/3Subtle Warning
It carries a mild warning against potential negative outcomes of unnecessary action. 
Her temper has cooled down; let's leave well alone now.