We have fields in our Lead records that are populated by a form. The same fields exist in our Contact records but the data may be different. When we convert Leads, if the fields in the Contact record have data the new data in the Lead record DOES NOT over-write the data in the Contact record as we would like. 

Is there a way to identify specific fields that can be over-written in the Contact record when a Lead is converted?  If not, any other suggestions for how to address this problem?