Complete transformation of main coins (3,6)
I believe the answer is:
sea change
'complete transformation' is the definition.
'main coins' is the wordplay.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'coins' becomes 'change' (I've seen this before).
'sea'+'change'='SEA-CHANGE'
'of' acts as a link.
(Other definitions for sea change that I've seen before include "Significant transformation of marine origin" , "Notable transformation" , "Profound transformation" , "Major transformation" , "Significant or unexpected transformation - on the briny?" .)