Dic: If it ain't broke, don't fix it

• We're happy with our exam system in Scotland, and as they say, if it ain't broke, don't fix it.
• He hasn't made a lot of changes to the team since taking over as head coach, figuring if it ain't broke, don't fix it.
• So long as they like our proposal let's not change it; if it ain't broke don't fix it.

1. Something that you say which means if a system or method works well there is no reason to change it.
2. Don't meddle with something that's functioning adequately.
   
   Cf. This folksy and deliberately ungrammatical expression dates from the mid-1900s. For a synonym, see leave well enough alone.
   cf. Principally known in North America; the British equivalent is let well alone.

...... http://idioms.thefreedictionary.com/if+it+ain't+broke,+don't+fix+it,
http://www.answers.com/topic/if-it-ain-t-broke-don-t-fix-it