How to add Data source attribute as Key when exiting transactions and matches exist in the profile?

Summary:

We want to add a new data source attribute and assign it as Key in order to avoid duplicate load of transactions. But, there are already transactions loaded and matched (many manually) and we would like to avoid deleting them and starting over. We get this error: "Could not create attribute xxxx as Required/Key in data source xx because there are transactions already in the data source

Content (required):

error: "Could not create attribute xxxx as Required/Key in data source xx because there are transactions already in the data source

Version (include the version you are using, if applicable):