hi Ahmed,
As i mentioned please make those fields opitional only. dont make them mandentory unless you have specific posting keys or GL A/C where you want to activate the Ref keys becuase at the time of clearing those old open docs system will check the Ref keys availabiliy if you make those fileds mandentory.
Please do test this senario in QA/Test system before doing any changes in PRD enviornment. Do the changes only for the relevent GL/posting keys.
Regards
GayaniP