Extending fiat transaction log to support cryptocurrency
Any possible impact to original currency amount conversion that i need to look after for when i increase the precision? 


Yes.  

Carefully review your calculations against the table here: Precision, scale, and length, because you can have loss of precision when multiplying decimals:


  The result precision and scale have an absolute maximum of 38. When a
  result precision is greater than 38, it's reduced to 38, and the
  corresponding scale is reduced to try to prevent truncating the
  integral part of a result.