Unfortunately, once a custom field is created, you cannot change its type. There's an improvement regarding this request being tracked at https://jira.atlassian.com/browse/JRA-19312 but it is already closed as "Won't Fix", so it won't be addressed any time soon.
What you can do is create a user picker custom field, and then migrate all of the data from the existing custom field into the new field using the bulk editing tool. After that, you can delete the existing custom field, and rename the new custom field to what the one you just deleted was.
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