Hi Rajshree
Changing the currency is not advisable as these would effect the G/L accouts created, i don't know whether you have created group currencies and local currencies. It's better you can add new entries in OY04 with USD, but before doing so discuss the same with FI consultants and make sure no previuos postings are done, because changing the currency decimals now shall make previous postings as invalid and report generation by FI will fall as an issue. Please revert in case of queries.
Sajish