As outlined in this blog, you can't simply change the currency key defintion on decimals in a living system, when you have done allready postings (the more, the worse, so to say...)
The reason why you can't do so from a day 'X' onwards without having changed also history is selfexplaining: you will create the more the worse inconsistency, since you have applied the change from a key date onwards, ignoring nevertheless the fact that postings have been already made before, thus one and the same currency key has either a increase or decrease by factor 10 per decimal that you change either up or down from that keydate onwards - clear so far, I would assume?
Now: as also outlined in this blog, you do have variants:
Variant 1: Create your own currency key as a new name with your definition - absolutely free for you, if you are not bound to any restrictions why you can't do so.
Variant 2: (if only few postings are affected, say some 100 documents) take back the postings. Change the decimals defintion in a test system. Replay the posting under the new defintion in that test box. Check briefly. If suiteable, and tests were 100% fine, you may consider to do the same on Production.(not recommended by SAP)
Variant 3: If you are bound to history or other rules: SAP SLO currency key conversion (also known as 'decimal conversion) or SAP SLO currency conversion.
See also SAP note 434349 on this subject.
Hope this helps, Uwe