Not knowing much about Shelby's--what do you mean by "69's were converted to 70's"?
Short version - and a really strange story (don't know if it has even been done before or since)
Left over, new, unsold 69 Shelby's (lots of drag pak cars) and late production cars were re-serial numbered and sold as 1970 models. Feds over saw the conversion of the 69's being done in the field and some minor updates were done to them to match the 70 features.
If tried today the lawyers and politicians would surely have a field day