Convert Probability(%) field in editable dropdown

Hi,

I want to convert Probability(%) field in dropdown and editable. I remove default formula from studio and make changes in vardef file as below.

$dictionary['Opportunity']['fields']['probability']['type']='enum';

$dictionary['Opportunity']['fields']['probability']['options']='sales_probability_dom';

$dictionary['Opportunity']['fields']['probability']['enforced']=false;

But when I select value from probability and save the record, probability value save as blank.